The AMD 6850 is probably what you might consider. It's nearly 20 times more powerful than your current card based on the gigaflops. If you can find a great Black Friday deal, consider the 6870 instead, of which is nearly 30 times more powerful. If you want to go the Nvidia route, then the 550 Ti or 560 are your best bet (the latter only if you can find a great deal).
However, what PSU do you have? If you have a weak PSU, you won't be able to get a significant upgrade. If your PSU has no PCI-express power connectors available or is below 30 amps or 400 watts for the +12V rail, you won't be able to run the higher end cards.