We'll also need your power supply info including, wattage, 12V rating in amps, and how many 6-pin PCIe connectors you have if any. If you're lacking PCIe connectors, how many 4-pin molex connectors do you have extra in your system?
Assuming your PSU is good enough, with your current system and budget you can get into a decent GTX 560, or a budget oriented 560 Ti. On the AMD side I'd look for a nice used HD 5870, or a new HD 6850. All of these cards will require 2 x 6-pin PCIe connectors or 4 x molex connectors. The AMD card will probably run around $150-175, and is the best deal performance per dollar wise. It offers the equivalent of GTX 560 Ti ($200-250) performance at GTX 560 ($150-175) pricing.