Most graphics cards use the AGP slot (and newer ones also use PCI express now I think - which isnt the same as PCI) - which yours doesnt have.

PCI slots are those 3 white slots at the bottom of your case of which one of them you've already got occupied with a modem? You use these slots for pretty much anything except graphics cards! You can get PCI graphics cards but they are not that common and I dont think they are all that powerfull because of the limitations of the actual slot.