I got an XFX 8800GTX yesterday and Ive been seeing how much i can get out of it. Ive clocked my conroe to 3.6GHz which a lot of people can reach so it may help you decide whether to buy 
i have to say before i start, coming from a 7950gx2 the image quality is far superior and easily matches if not surpasses the Ati cards.
System:
Asus P5WDH
Conroe 6600 @ 3.6GHz
2GB Team xtreme
XFX 8800GTX
3D mark 06
Heres default run with GTX at stock (575/900) quality settings in driver.

Bumped it up to 630/950 quality settings in driver.

Pushed it hard 660/1030 quality settings in driver.

Impressive but ive seen people getting 12000 with similar system.
Time to run some tests with driver set to performance
Same clocks as last time 660/1030 but with performance setting.

ok, well this seems to be my limit, when going higher it doesn't artifact at all it it will just freeze solid.
660/1070 my max with performance setting.

Ok, one last test, ive raised my Conroe to 3.8GHz..

Hope this helps!

i have to say before i start, coming from a 7950gx2 the image quality is far superior and easily matches if not surpasses the Ati cards.
System:
Asus P5WDH
Conroe 6600 @ 3.6GHz
2GB Team xtreme
XFX 8800GTX
3D mark 06
Heres default run with GTX at stock (575/900) quality settings in driver.

Bumped it up to 630/950 quality settings in driver.

Pushed it hard 660/1030 quality settings in driver.

Impressive but ive seen people getting 12000 with similar system.
Time to run some tests with driver set to performance

Same clocks as last time 660/1030 but with performance setting.

ok, well this seems to be my limit, when going higher it doesn't artifact at all it it will just freeze solid.
660/1070 my max with performance setting.

Ok, one last test, ive raised my Conroe to 3.8GHz..

Hope this helps!