Any PCIe graphics card will work - it just depends on how much you want to spend and what sort of games and settings you want to play at. What power supply do you have as that will possibly limit use of higher end cards.
What is the specific model name of the PSU? Is it a high quality one or a cheap one bundled with a case? You will be limited to a lower end graphics card such as an HD6670 or similar card which doesn't need a 6-pin power connector.