first of all here seems too many problem but i think all of them is connected to one reason.
problem 1: (dirt 2)
There is many stuttering in the game. when i play it starts to stutter and immediately it gets very fast grapchis. afterthat again it starts to stutter and grapichs gets very fast. i tried to get a fps but i could not. I can not get a benchmark report on Dirt 2. I complete benchmark test in the game but ne result is shown.
the stuttering and getting fast is shown with almost all games. and in crysis(for example), there is no v sync enabling option in graphic options. so some tearings is shown during game.
low fps and performance. the computer systems that are very similar to my sistem gets better points from tests like 3dmark06 although i overclocked my sistem.
* all drivers of my hardwares is all updated. catalyst 10.1
* I do not even think that system is getting hot too much. all tempatures is all okay. fans works with their full speed.
The thing that stands out to me is that you have 2 sticks of 2GB's or ram on a board that uses tri channel memory.
After reading my own manual, which is an ASUS P6T, I guess it can go into dual channel mode, and yours will likely too. I'm still curious if that could cause a problem. You also may want to make sure you have the sticks of ram in the right slot. We've had people here get confused on the proper slots on these boards, and oddly, they systems run, just very poorly.
Another thing to check is the memory speeds on that 5870. The cores on these cards overclock well, but the memory does not. You might try the memory at 1200 or even 1150 clock speeds.