The 4850 runs at a stock of 625Mhz core and 993Mhz GDDR3.
I have an A64 X2 4400+ @2.4Ghz with a 4850 running smooth. I don't know if a 8800GT would do better, i don't have one to do that, but i can tell you to check in the CCC, under 3D settings what you have on and what not.
If you run with AA or AF in games, and have them on in the CCC, your card will be doing double work.
EDIT: Spelling XD