[Off-topic] sakai4eva:
Hmmm... talking from experience here, so bear with me.

1. New GPUs are hot. I mean they suck the juice outta you. I mean, they run at high temperatures and require a high wattage PSU. That being said, usually they will have six/eight pin, sometimes double(!) power connectors. My 6950 is running two six-pin connectors. Also, almost all PCI-E ports are x16, and I'm sure the above posters have done their legwork for you. Since you're looking at a single card solution, getting something top-of-the-line, or thereabouts will do. NVIDIA/AMD doesn't matter, just get the BIGGEST, FATTEST, FASTEST (first two doesn't really jive with the last, right) card you can afford. This saves money, and gives you more bang for your buckeroos. They're BIG too. The size of my 6950 is drawn in the red box.

2. Smart move on getting a new case. Refer to no.1 for a good reason why I'm saying this. Here's a bonus tip; try getting it for FREE. Recycling centres, friends, even random custom PC shops might have them. check around your area. #1 important criteria for a PC-case = size. Bigger is better, and if it's the size of your room, huzzah! DISCLAIMER: my parents own a shop. I snatch free screwdrivers, screws, the occasional case (when mine get too dusty), cardboard, plastic and duct tape (explain later) and internets (actually, my own connection is faster :-|).

3. Air-flow + cooling. Almost split this to two different topics, but WTH. The are drawn in lime green is the size of a decent after-market cooler for your CPU. Depending on over-clockability, a CM Hyper 212 Plus (really cheap, like >USD20 cheap) will give you about 20%+ in temperature leeway. Good for ekking out just a bit more. WARNING: check our your processor slot first to ensure compatibility. Now, on to cooling. Notice the red and blue arrows? With the CM H212+, your airflow will change to that (you can do it the other way around, but you'll melt the PCB when playing Crysis). Oh, blue is cool air, red is hot air, but I guess that's intuitive enough, right? While it will seem idiotically retardedly stupidly imbecilic moronic dumb to let one component's hot air flow into another component's cool air intake, you should notice that purplish (borderline gay, but who am I to judge how your PC does things) block. That is your custom cooling solution, aka using plastic/cardboard + duct tape to change the airflow. Be creative, and just remember that hot air flows upwards, while being mindful of cables.

4. PSUs. Here is where $#!@+ goes down. No.1 advice? Don't skimp. Components might sound cheap, but they are cheap for a reason. To cater to my 6950, I bent over and bought a 650W 80+ silver PSU. From your posts above, I surmise you understand about wattage, but you need to learn about the 80+ certification thing. It just means that when you're PC is running low loads, only 20% of the certified power draw is being used at any time. Good for energy efficiency because I leave my PC on 24-hours.

5. CPUs. @DirkGently pointed out that you shouldn't get an über-powerful (sorry for the ümlauts) GPU without getting an über-powerful (I'd apologize again, but I'm repeating myself) CPU. So... get one? NO! Over-clock that B!@+c#! (Note that the last exclamation mark is a mark of exclamation) My Phenom II x4 945 is showing its age, but my-oh-my it's ageing more gracefully than Lindsay Lohan.

Before I end, there's a few things I need to comment on; first being the lack of info from your side regarding the "proper" use of your PC. Specifics like, gaming at what resolutions with what usual effects on, or just simply going on Farmville to further your goals of world domination via virtual agriculture should suffice. We do not wish to learn about your h3n+@! gaming titles. Also, listing the exact and entire specs of your prior PC will go a long way towards organizing a proper refresher on your PC.

That should be about it, and thanks for indulging in my brief, profanity-laden, typing spree. Have a good day, and if this REALLY kicks off (I'll know when I get a thank or two here) I might post pretty pictures of how my monstrosity looks like.