You could keep the 4870 and look for another cheap on ebay - I wouldn't pay more than 70-80 dollars and I would only do it with the 1 GB version, not the 512 mb. That would give you good returns but no direct X11 - so possibly not as good graphics in new games but cheap (60-80 bucks total).
Or you could sell your card on ebay and get a new card. Which new card would depend on your system specs screen and such so if you post that I'm sure we could give suggestions. Right now $250 bucks buys you a 5870 or a GTX 470 if you want to spend it all