It depends on what kind of expansion slots the motherboard has.
If it's AGP, the 4650/4670 are nice; the 3850 is probably a little bit better card if you think your power supply has enough juice and connectors to handle it (be warned, it requires an 8-pin or a 6+2 pin PCIe connection).
If you've got a PCIe x16 slot, you can pretty much get any card you want, although the really high-end cards are probably a waste because the CPU will be the limiting factor. I'd only go with a 5xxx or 4870/90 if you plan on using the card in a future system.
Also, whatever you do, don't get a card with a regular PCI interface. They do make versions of a lot of otherwise decent cards in regular PCI versions -- but the transfer rate of the slot itself is several times slower than a PCIe x16 or even an AGP slot, so you're not going to get very good performance. Reserve regular PCI slots for peripherals like ethernet cards, controllers, external devices, maybe a sound card -- but never a video card.