In your case, just make sure that the graphics card that you are eyeing is made for the PCI-E interface. As well, alot of the newer cards, atleast ones that are worth gaming with, take up alot more power than most stock cards that come with pre-built pc's. If you are planning on upgrading to a gaming card, make sure your PSU is capable of handling the load. Mind posting us a nice list of your current specs, including your PSU? That is, if you know it
I will be glad to help you if you have any more questions.
Those of you wondering where the hell I came from, Im new here, but I have custom built about 3 gaming systems, so I can definately help with your computer problems or questions