Q:

High Magic Find Act I or no Magic Find Act III / IV? 

The title pretty much sums it up.  A friend are lvl 60 and playing in inferno and we have gear to survive on act 3 / 4, but have high Magic Find gear that only works in act 1 (and barely works in act 2), we were just wondering what would be better to get better drops.  Any ideas? 
Thanks in advance! 

A:

Short Answer
Higher acts are better, unless you can kill at least twice as fast in the previous act. 
Which act you run should depend on what your current goal is. 
EDIT: With the addition of Paragon levels in 1.04, Magic Find slowly becomes an inherent stat as you accrue Paragon levels.   To that end, it's impossible to lower Magic Find past a certain point.   To that end, this answer gets simplified to completely depend on your kill rate. 

Only iLvl 63 gear
Description: You want the best gear that you can get.   Anything below this is dog food.   Only the cream of the crop for you.   If if's not an Infinity+1 sword, your time has been wasted. 
This one depends entirely on your kill rate.   Act 2 provides almost twice the chance of iLvl 63 gear than Act 1.   But if you kill only half as fast, you'll get roughly the same amount of items.   That's your metric.   If you can kill 100 demons in Act 1 in the same amount of time that you can kill 50 demons in Act 2, you want Act 1.   But as soon as you can kill 51 demons, switch to Act 2.   The same goes for Act 3. 

For historical purposes, the previous answer is being kept below. 

Long, drawn out (with math!)  Answer
Preface/Basic data:
Since Blizzard published the fact that Magic Find increases the quality of your drops, as well as the quantity, Magic Find plays a much more important role in your required stats than it used to. 
Blizzard has also published the general drop rates for the highest level equipment:

Inferno – Act 1
      iLvl 61: 23.9%
      iLvl 62: 12.6%
      iLvl 63: 4.8%  
Inferno – Act II
      iLvl 61: 23.3%
      iLvl 62: 18.6%
      iLvl 63: 9.3%  
Inferno – Act III and Act IV
      iLvl 61: 27.1%
      iLvl 62: 21.7%
      iLvl 63: 16.3%

Based on this data, the chance of getting Inferno-class gear is:

Act 1: 41.3%
Act 2: 51.2%
Act 3: 65.1%

Note that this percentage does not affect if the gear is magical or not; that's where Magic Find comes in. 
Goals
Most Inferno-class Gear
Description: You want gear.   If you punch a demon, you want it to explode with gear.   If it's not raining gear, it's because that demon you're eyeing just hasn't exploded yet.   Better change that...
You're going for quantity over quality for this goal.   Simple and easy is the name of the game.   The more you kill, and the faster you kill, will net you more gear for the same amount of time investment.   You want the highest act that you can kill things easily.   If Act 1 is a yawn, and Act 2 gives you no trouble at all, run Act 2.   Same with Act 3.   This will let you gather as much gear as you can hold, over and over and over.   Magic Find is not a factor for this goal. 
Most Affixes Inferno-class Gear
Description: You want gear.   The more stuff it gives you, the better.   Doesn't matter if it's useful or not.   You want the glowing sword that sings, so you can show it off to friends.   
This one's a little bit of a moving target.   Your goal for this is to try to get as much 6-affix gear as you can.   To this end, Magic Find plays a very large part in what you want.   Run Act 1 with as much Magic Find as you can possible carry and still have no problems at all.   Now, here's the tricky part: If you find you are getting more legendaries than you're really wanting, that means you have too MUCH Magic Find; you want just a little less than that, to get the 6-affix rares.   This is the sign that you should move up an Act.   Switching out gear to be more survivable will take care of the extra Magic Find, and the higher act will garner you better chances of Inferno-class gear. 
Only iLvl 63 gear
Description: You want the best gear that you can get.   Anything below this is dog food.   Only the cream of the crop for you.   If if's not an Infinity+1 sword, your time has been wasted. 
This one depends entirely on your kill rate.   Act 2 provides almost twice the chance of iLvl 63 gear than Act 1.   But if you kill only half as fast, you'll get roughly the same amount of items.   That's your metric.   If you can kill 100 demons in Act 1 in the same amount of time that you can kill 50 demons in Act 2, you want Act 1.   But as soon as you can kill 51 demons, switch to Act 2.   The same goes for Act 3. 
6-affix iLvl 63 gear
Description: You want the best of the best.   It's an Infinity+1 sword, but it glows, sings, and if you let it, it'll dance, too.   You show this off to other Nephalem to make them wish they were you. 
This is a combination of the previous two goals, and as such, depends on both the amount of Magic Find you can stack, and your kill rate while wearing said gear.   This one's harder to quantify, as Blizzard has not released the chances of 6-affix items dropping, most likely to discourage farming the same thing over and over.   My gut says that the higher acts have a better chance of dropping 6-affix gear, but I have nothing to corroborate that.   So, you'll need to balance this yourself. 
Disclaimer: I'm using some hypothetical values to demonstrate my point. 

Using the assumption that all enemies (regardless of act) have a 5% chance of dropping a 6-affix item. 
You can run through Act 1 with 150% Magic Find
You can do Act 2 with 75% Magic Find

With these values, you get:

Act 1: 5% (hypothetical) * 1 (base) * 150% (Magic Find) * 4.8% (Act 1 iLvl 63) = 0.6% chance of 6-affix iLvl 63 gear. 
Act 2: 5% * 1 * 75% * 9.3% = 0.81% of 6-affix iLvl 63 gear. 

So, using that premise, you have roughly a 25% higher chance of getting what you're looking for, with half the Magic Find gear.   If your kill rate is equal or just slightly slower, Act 2 is where you should be.   
If you can add extra Magic Find and keep your kill rate constant, do it.   More Magic Find = better chance of finding 6-affix iLvl 63 gear. 
Notes
This is making the assumption of the drop rate of 6-affix gear is equal throughout Inferno.   If there is any change in drop rate (and we know what that change is), you can use the formula above to determine what your chances of the item are.   You can use your kill rate to determine which act you should be running at that point. 
I am also making the assumption that the affix chance is entirely separate from the item level drop rate.   If there is any correlation between them, this formula is completely wrong and will require either re-working or starting from scratch. 

A:

Magic Find has two major changes on the drops:

It increases the drop rate of magic, rare and legendary items. 
Also affects the chance of getting more prefixes (6 prop items vs 5 or 4 property versions), thus allowing for a much greater chance of getting decent or good items. 

Act 1 Inferno has a lower drop rate of iLvl 63 items which has the best potential rolls due to higher base stats.  However it also has far lower monster hp which allows for a faster clearing speed compared to the later acts.  One caveat is that I've read that higher level blacksmith plans only drop in the later acts, if you are interested in a chance to get those (I don't have a citation or proof for this). 
Another factor is easy access to elites; since Inferno farming is usually getting 5 NV stacks and going for elite kills rather than a full clear of white mobs.  In this aspect, Act 1 is fairly efficient as you can do Cemetery + crypts, Festering Woods, Cathedral 3 and Leoric's Manor Courtyard with high incidence of elite spawns. 
As such, I'd recommend farming in Act 1 with high MF stats. 

A:

So to answer this question, it helps to understand how Magic Find, and more generally, loot in Diablo 3 works. 
In short, when a mob drops loot, the game first determines what he'll drop -  the base items.  Let's say, a level 32 crossbow, a level 34 ring, and a level 35 shield.  Your magic find has no impact on this.  Only the level of the foe dropping the loot or chest being opened matters here.  The game then does a second roll to determine the quality of the items in question.  The ring rolls up blue, the crossbow rolls up gray, and the shield rolls up white, for example.  This roll is impacted by increased magic find. 
So what does this mean for you?  High Magic Find will never increase the quality of loot you acquire.  If your goal is to find drops that are useful to you and allow you to progress in the game, increasing MF will only help insofar as you are able to do so in content that is capable of dropping useful items to begin with.  In general, before level 60/Inferno, the best advice is to wear as much MF as you can without hurting your ability to progress.  It is pretty much always going to be better for your ability to gear up and your ability to get to level 60, to sacrifice some magic find in order to be more effective in combat.  MF will increase the quantity of drops, which can be helpful if your goal is to acquire lots of items to melt for crafting mats, or to sell on the auction house.  However, in general, this is not the best use of your gaming time before level 60, so unless you're just spinning your wheels while waiting for a friend to catch up in level or something, I can't recommend it.