I agree with MrRipley, Smokey. All of them should give you a nice performance boost as compared to your old 8500GT, so the real question is one of cost. Of the three cards you listed, I'd say the 8800GT is probably the best overall value, dollar-for-dollar. However, the way you play may change the equation slightly. Questions that could potentially affect the way you might go:
1.) Do you typically play at higher resolutions (say 1280 X 1024) or above?
2.) Does your motherboard support SLI, and do you plan to ever use it?
If you never play at higher resolutions, I think the 9600GT would be the absolute best bang for your buck option, since they are currently running in the ~ $175 range right now. If you intend to go SLI, but want to keep it as cheep as possible, this is probably another reason to get the 9600GT.
However, if you don't mind paying the extra $30 - $40 or so for the 8800GT, it'll probably do you right. Especially if you crank the resolution up, turn on the AA, etc., etc.
One last thought - if your board does support SLI, and you are flirting with the notion of using it now, or down the road - be sure to give the size of the new video card's cooling solution some thought. Most of the 8800GT, and 9600GT cards use a single-slot cooling solution, whereas the 8800GTS & 8800GTX cards typically have dual-slot cooling solutions. Depending on how your motherboard's slots are configured this could potentially be an issue - because the cooling fan on your first video card could block slots when you decide to add another card in later.