Graphic card

Hello and welcome,
Giving a guess that you have a good CPU so you have a decent MOBO that supports PCIE 2.0 X16. So any PCIE 2.0 X16 card will fit properly and work fine.

Now it depends more on your needs, if you're hard gaming you can grab any high end GPU from nVidia or AMD such as GTX 570/HD 6970.

If you're just watching HD movies and doing some visual effects a low end GPU such as HD 6670 will be a good choice.