How much are you wanting to spend? You can probably get something for your current 8600gt on ebay, maybe even your 8500gt if you've got that, that can offset the price a little. If you look on newegg.com, they've got the 8800gs which can be had for about 150. Spend about 170 though and you'll be able to get a 9600gt, either of those should be way better than what you have, look around google and search for reviews, they should give you an idea what kind of performance you could get.
Best card for budget gamer, your eyes will literally fall off when you see the kind of improvement over the 8500GT. Think of it like the 8500GT is an engine that can push a car to maybe 60mph tops, while the 8800GS is the kind of engine that is able to do anywhere between 400-450mph and still has potential for a decent overclock that will take you even further (8500GT overclocking is amazingly pointless)
well i have the same question so i will post it here even though it is outdated
i have an 8500 GT and am upgrading but now the geforce 9 series is out so what should i get
i am willing to spend up to 220 but would really prefer to keep it under 170 b/c then i can get a tv card instead of only having enough money left over for a headset