Well I'm guessing you don't have a PCI-E slot, and the AGP versions are now slightly more expensive, but here's a 256 meg 6600GT.
http://www.novatech.co.uk/novatech/specpage.html?NOV-6625GT
You can knock a bit off if you only want the 128 meg version but in this case, I think the 256 is probably worth it.
http://www.novatech.co.uk/novatech/specpage.html?NOV-6625GT
You can knock a bit off if you only want the 128 meg version but in this case, I think the 256 is probably worth it.