possibly a 6800GT? if you can find a used 6800GT in FS/FT forum, or on ebay or something.
if not AGP 6800GS's are good cards too, roughly being the same performance as the original 6800GT.
if you wanna stick with ATI, theres lots of choice. there are numerous X800 cards for the picking. X800, X800GT, X800GTO, X800XL, i cant remember the specs of the top of my head though
lets have a look see
X800 is 8 pipes i think
X800GT is also 8 pipes,
X800GTO has 12 pipes
X800pro has 12 pipes as well
X800XL has 16pipes
theres also the X1600pro which can be had for under £100 but im not sure that cards really a good deal
you may want to wait for some one who knows more on the ATI side of things, they released too many variations for me to keep up with lol