Juuuust wondering if it is advisable to turn off fastwrite in ati drivers and/or changing agp speed from 8x to 4x if you want better performance from certain games ??

For the most part you would find that most newer games would perform better with fast write and 8X,process speed is usually the determining factor to maximize a games enjoyment.
Older games may not be written to take advantage of fast Write setting.
It is always best to go to the site of a Game's manufacturer and read the tech info.Barring that you just have to experiment with each one.

Ive always heard that keeping fast writes enabled with ATI cards causes instability. Its been true with my systems. Deffinately enable 8x.

8X is signifigantly faster than 4X by 1-2%. It used to be that Fastwrite and Sidebanding were bad for nVidia boards but I thought ATI people could run them. If things are running well with them off, turn them on and see if you get any new problems. They are supposed to add small performance gains so if it works with them on, then leave them on.

I have nVidia and I run with 8X, Fastwrites and Sideband Addressing. It's not going to break anything to test it.