There's your basic PCI, which yours has. There is a new PCI version called
PCI-Express. If you had that you'd know about it by the galaxy sized void
in your wallet.
I don't think your machine actually has an AGP port, which is what you
really want for graphics cards, so to be blunt; graphics performance will be
a fraction of a machine with an AGP port.
What sort of games are you looking at playing? If you want to play modern
stuff like Half-Life 2 on decent quality settings, it'll probably be easier
and cheaper to get a new computer. Other wise you'd be looking at upgrading
the motherboard, probably the RAM and CPU too. If the Dell box is even
upgradeable like that.