coolpurplefan - HDR, Purevideo/AVIVO and SM3 are entirely useless in this conversation - the game this guy is playing came out before HDR/SM3 were used, and Purevideo/AVIVO are for video playback accel and have no impact on gaming.
What IS important is the technical specs of your 8500 versus the the 6600gt, 7600gs and 7600gt cards:
ATI 8500 * = 2 pixel procs & 2 vertex procs - 275 mhz core, 550 mhz ram (* directx8 only)
6600GT = 8 pixel procs & 3 vertex procs - 500 mhz core, 1000 mhz ram
7600GS = 12 pixel procs & 5 vertex procs - 400 mhz core, 800 mhz ram
7600GT = 12 pixel procs & 5 vertex procs - 560 mhz core, 1400 mhz ram
The 8500 is totally underpowered compared to any of these cards and only supports directx8. The 6600gt is MUCH faster than the 8500, but obviously at a disadvantage against the 7600GS in terms of raw processing power. The clockspeed advantage of the 6600GT narrows the gap some but the 7600GS is still a faster card by at least 25% depending on the app. The 7600GT has the same basic processing power as the 7600GS with much higher clocks.
I would say get the 6600GT used in the $50 ballpark since it will get you the most bang for the buck in terms of performance over your current card.
Otherwise just jump into a 7600GT :
http://www.xpcgear.com/evga76gtn615.html - $115 + shipping). Most of the 7600GS AGP cards are $100+ so why not jump into the better card for just a few bucks more?
Here's a reasonably priced used AGP 6600GT ($55):
http://forums.anandtech.com/messageview...atid=45&threadid=2047385&enterthread=y
Watch out for the 6800/6800LE cards - basic configurations vary wildly and I found a number with fewer processing units, slower clocks and/or slower RAM compared to the 6600GT. SOME of the 6800's or 6800NU cards will be faster but their price, even used, puts them in line with the 7600GS which would be faster still.
<lots of crazy edits after I got more info on the various chipset's architecture stats and did some price research>