First off, I assume you mean for gaming.
It depends on your resolution. Assuming you're a normal gaming resolution (anything between 1680x1050 and 1920x1200), the best cards for the money would be the Nvidia GTX 560 TI or the ATI HD 6950. For higher resolutions, the 6950 might be better because it has the possibility to be unlocked to a 6970. Regardless, they're both very good. Both of these cards are around the $250-$275 range, and can run current games at max settings with the listed resolutions.
If you want something a little cheaper, the GTX 460 is a very nice card that you'll be able to find for around $150-$160.
Keep in mind, you'll want to make sure you have a 500w PSU minimum for any of these cards.
With your processor, I would personally be inclined to the 560, but you can't go wrong with any of these. A GTX 570/580 will futureproof you even further, but the price only makes it viable to those with huge resolutions. Besides, you always have the option to SLI/Crossfire any card you buy today.
Good luck with your purchase!