ATI's latest GPU the 2900XT was designed to compete against the 8800GTS 640MB/320MB, and it did that very well, beating it in most benchmarks, and with the new drivers its right behind the 8800GTX and closing fast, perhaps it might very well be even more powerful than the 8800GTX once they fix all the issues with it, making it a must buy price vs performance card.
8600GT/GTS, don't even bother, the only good thing about them is DX10 and you can't use DX10 features yet, and for the price vs performance its a mustn't.
8500GT, not to sure, but that card performance is less that than the 7600GT, and the 7600GT is only managing to cope with todays games, so unless you like to play games at low detail, than the 8500GT is a mustn't.
Radeon 2600XT, I haven't seen any benchmarks, but it will compete against the 8600GT. It will most likely come out on top, and with the same price (I'm guessing so don't get me wrong). I would recommend wait a little for the 2600XT benchmarks before making your decision.
All-in-all, don't even bother to get the 8500GT, 8600GT/GTS. if you can afford it, than get the 8800GTS (only if it comes down in price), if you can afford a 2900XT, than by-all-means, get it, you will own a lot of people hear. If you get a 7600GT, thats good, as it offers one hell of a bang for your buck. 7900GS, like $100 more where i live but offers almost double the performance.
I don't know, your best to wait it of for just 1 more month. And yes i know companies a continually bringing out newer and better products, and we are gonna keep urging you to wait it out a little more, but can 1 more month really hurt?, I've been waiting almost a year to upgrade my 512MB of RAM to 2GB, and now that the RAM prices are ridiculously cheap now That might just happen, I might even be able to pick up a good pair for the same price as what a crap pair cost a month ago.