In D3 and Q4, the 6800 unlocked to 16x6 will beat the 6600GT by some margin.
I score about 40FPS at 1024x768 with 4xAF on my 6800 with a A64 3000+
Considering you have a XP 2600+ you'll probably have to drop AF to 2x, otherwise Q4 and D3 will be fine.
If you feel 6800 is good, go get it, don't wait for opinions here as the 6800 is always criticised for some reason or the other
I score about 40FPS at 1024x768 with 4xAF on my 6800 with a A64 3000+
Considering you have a XP 2600+ you'll probably have to drop AF to 2x, otherwise Q4 and D3 will be fine.
If you feel 6800 is good, go get it, don't wait for opinions here as the 6800 is always criticised for some reason or the other