any PCI-E 2.0 or 1.0 card will work
an ATI 5570 or 4670 will probably be a good fit. I would also check ebay and see if they have any Pentium D chips at a decent price. The Dell OptiPlex GX520 can take up to the D-840.
Getting another 1GB will also help.
*I forget that PC's case but if its a slim line then you need a low profile card.