GeForce 9600 GT, 9800 GT or Radeon 4850 if you don't want to spend too much (the cards are placed in order from cheapest to most expensive, but none of them are truly expensive).
You just buying a graphic card? You might want to check whether it's worth it to buy a new GFX for your old computer (ie. if your CPU is old a new GFX card might not be worth it).
Shopping online is cheaper usually. I live in Australia so I can't give you price references but it's not that much cheaper here in Australia.