If you're partial to Nvidia cards, check out the GeforceFX 5600's for some budget performance. The ti4200 and ti4800 cards are still quite decent for the price, too, if you aren't fussed about having a DX9 card.
If you're more of an ATI fan, you could look at the Radeon 9500's or 9600's for some low price, high-performance action. Personally, I'd say the Radeons are a bit of a better deal right now, but some people still prefer the Nvidia cards.