I'd go with this for your system
POWERCOLOR AG3650 $59.99http://www.newegg.com/Product/Product.aspx?Item=N82E16814131173
If you don't mind going on eBay then you may want to get an ATI X1900 or nvidia 7900 series AGP card. Of course those cards are a bit power hungry so make sure you have a decent PSU. Overclocking your CPU would certainly help to make things playable. If you have a Barton Athlon 2600+ (I think there was also a thorton, ie 256k cache disabled) then you should be able to overclock it a bit. If it's the Thoroughbred, not so much.