Well, as far as I've read the 8600GT is kind of a dissappointment, due to 128bit memory interface, so my guess is that the 8500GT will do no better.
I think it's ok for mediocre gaming on normal settings, but don't expect any uberframerates in the newer games.
For that you really need the 8800 series or the x2900 card.
I think I read somewhere that the 8600GT performs about the same as the X1950XT, which is the card I have, and it's really nice. Most games can be played on higher resolutions, but with details to medium and no AA and AF I think, tho I'd have to check at what settings I play games, varies per game.