I have a 9600GT, HD 3850 512MB and the 8800 GTS 320MB (G80).
All three are overclocked. The 9600GT wasn't much of an overclocker, barely hitting 720/2020
from the default 650/1800. Pretty much the same results with both a Palit and a PNY G94.
The 8800 GTS 320MB hit 650/1500/2000 and the HD 3850 hit 830/2250, both stellar.
I play most games at 1152 x 864 4AA and 16AF all settings on high, except when testing Crysis.
Overall the HD 3850 is fastest, followed by the 8800GTS with the 9600GT bringing the rear.
There isn't much difference between the three. HD3850 is usually about 5 fps ahead of the 8800GTS and ten ahead of the 9600GT. Except in Crysis, the HD3850 is a good 15-20 fps ahead of the other two using the Xtreme-G 8.7 with no CCC (atitraytools), The Nvidia cards are using the Xtreme-G 177.41.
Pretty much the same results in other games, COD4, GRAW2, GRID, Test Drive Unlimited,
Oblivion, GTR2, ARMA, COH Opposing Fronts. At the above mentioned settings all of these cards hammer out good framerates, staying above 60fps minimum and often much faster.
I am not sure about higher resolutions, may or may not make a difference in the order of performance.
If budget is a concern, I got the Asus HD3850 512MB for $62.37 shipped after $30 MIR.
If I wanted a little more performance the 8800GT 512MB (G92) is down to $109 before shipping at the Egg, a good deal. Still, that's about $117 shipped for the 8800GT, almost twice as much as the HD 3850 but nowhere near twice as fast. Straight up you can find the 8800GS for $84, and the HD 3850 for $89.
It's a great time to be a computer gamer