[Galaxy S I9000 Q&A, Help & Troubleshooting] curioct:
Firstly gaining root access to access the hosts file is SIMPLE

Superoneclick v1.6.5 roots every sgs I have ever come across (personally I have rooted 4 different ones now) and I have used it to root a xperia x8 as well

I would then recommend downloading the busybox installer from the market and letting that install the latest stable busybox.

then ADFREE also from the market will modify the hosts file and automate the process of keeping that up to date.

on the choice of z4 my only experience was of that not working at all for me so that's all I can add to that.

personally beyond that I use odin to flash the voodoo lagfix and sound mod kernel with CWM and I love that. It's brilliant but I recently did someone elses SGS and it didnt want to play so I used the speedmod kernel with CWM instead. The quadrant scores aren't as high but it seems stable enough and quadrant isnt everything.

CWM will effectively let you back up the whole rom but in order to make effective backup's a simple rooted phone is enough CWM is not a requirement. With something like titanium back up from the market you can make all the back ups you want on a stock phone as long as it is rooted. Personally I do a CWM backup every couple or three weeks, but I do 2 different nightly backups with a weekly full backup scheduled on titanium and I use the titanium media sync to push the backups to my dropbox. This way I always have "belt and braces".

Might seem like overkill but I have around 350-400 apps installed at any given point. 5 lots of push e-mail etc etc etc.

(that should cover question 2&3 I think)

question4

Superoneclick will allow you to remove the the hack and the superuser and busybox programs.

you seem a little confused, there is a difference between hacking the phone for superuser (root) access (which is all you really need and can be done above with one click and no odin etc) and hacking the phone to modify the boot loader which is what clockworkmod is. This changes the recovery area of the phone.

typically when you do this you use odin to flash a new kernel which will include the clockworkrom recovery bootloader.

this is more risky (but still almost always recoverable from in my experience) typically the new kernels include a root solution so flashing a kernel does not require the phone to be rooted already. Usually flashing a new kernel will add extra benefits or features such as the option to change the filesystem that the phone uses to a faster one than the default used by android (this is usually called lagfixing).

technically I would guess that any of these things COULD invalidate warranty but in reality I doubt they would and you should be able to reverse almost any of them prior to returning a device under warranty. personally so far I have screwed up my phone and others a number of times and I haven't once found something that darkyy's ressurection rom hasnt been able to rescue me from. Worst case I flash that then reflash with a stock rom. (I've been able to do this when I have forgotten to unlagfix a phone when I have checked efs or repartition by mistake all sorts, I have had phones in the condition where the wouldnt accept any sim card and darky's has got me out of all these states simply repartition with the resurrection rom and bob's yer uncle)

Hope that answers your questions and helps you understand the differences.