FIND OUT if you have AGP
if you don't, it's probably not worth upgrading, since pci video is expensive and inferior quality. I'm guessing you do, however, so a good choice would be to look at these few cards from
www.newegg.com :
For value:
RADEON 9000 Atlantis 128MB
NVIDIA Gainward FX5200, 8X AGP, 128MB(64bit) DDR
For a bit more performance (still under $100 though, gotta love newegg):
ATI RADEON 9200 128MB DDR
PNY Verto GEFORCE FX 128MB DDR AGP 8X 5200 DUAL VGA TV-OUT (get this if you want or ever will want to run dual monitors, makes it so much simpler)