i believe that the 5900 is a 256bit card, so thats what you want out of the
two
however i would strongly suggest if you got that much cash, get either a
Nvidia 6600gt AGP(yes, they do agp versions) If thats still to expensive
think about getting a 9800 pro, don't waste your cash on a 9800xt as you can
easily overclock your card to perform like an xt model.
But if you can, definately get a 6600GT
heres a good site which compares most of the older generation cards( the
6600gt is a new gen card)
http://graphics.tomshardware.com/graphic/20041004/vga_charts-04.html#3dmark_2003
theres nothing wrong with the older cards but they will of course won't
perform as well as the newer cards under certain conditions eg. a 9800 pro
might get 200fps at 800x600 with the shaders and stuff turned off, but maybe
if you turn them on and take up the resolution to 1024x768 the 6600gt might
well perform better, but the spanking brand new games will perform better on
higher settings with the newer cards
hope that helped