OW.. a standard pci slot... ok - that makes it alot more difficult.
About the best your going to be able to do with a standard PCI slot is a HD 4350. Should handle SH3 without an issue, though the slot could still bottleneck you. One source I found was the following:
http://www.directron.com/ap4350512md2h.html
I can't speak to the seller though - never dealt with them.
On the positive side - with a PCI slot, you don't have to worry about your PSU rating nearly as much.