I am back with the 6600GT. After running the CS Stress Test it's a lot slower than reviews I've seen on the internet. I'm getting about 90 FPS, I've seen reviews of 6600GT with 130 FPS!
Maybe try a different driver? I am using 78.05
Edit: Now I've benched Doom 3. 63 FPS only, reviews say 80+ FPS.
Edit 2: Okay it appears I am CPU limited at 3.0 GHz. These reviews were usually 3.4 or so, I didn't think it would make such a difference. I know my CPU is 3 years old but it's still nearly as fast as the modern ones.
I benchmarked the HL2 stress test at 640x480 and got the same score as 1024x768, and then at 1024x768 with 4xAA and 8xAF and scored 83, only a tiny bit slower.
My Doom 3 4xAA 8xAF benchmarks match what I've seen in reviews.
So I guess this is a good thing, I can turn 4xAA and 8xAF on in most games with no performance hit compared to what I'd get without. The AA really does look heaps better.
Got a problem with Quake 3 though, when I enable AF it makes the textures look quite bad, ugly and obvious mipmapping levels. Maybe this was a driver bug I read about a while and it's been fixed... I will have to find out.