I upgraded from a 9800GT (same as an 8800GT) to a GTX460 (that I overclocked a bit using software included by EVGA). I noticed frame rate improvement of 2 to 4 times depending on the settings....Basically previosly with a playable framerate (based on settings I would get double frames with the 1GB GTX 460. However, the 460 was about 3 to 4 times faster if I turned on more AA and the such. So lets say I would go from 15 FPS on the 9800GT I would get like 50 to 55 on the GTX.
I think Tom's benchmark indexes help support this as they are FPS compilations of games
Gamer index http://www.tomshardware.com/charts/2011-gaming-graphics-charts/Gamer-Index,2673.html
Enthusiast index - http://www.tomshardware.com/charts/2011-gaming-graphics-charts/Enthusiast-Index,2674.html
So basically you will be able to turn up the details and probably get a better framerate then what you got with the 8800 GT on a lower framerate
If you keep watching on new egg you can get a GTX 460 1GB for like $120 after MIR (thats what I paid) Also make sure you do not buy the SE version.