look inside the case
if your monitor connects to a plug on the expansion slots on the back, you'll more than likely have a separate video card, otherwise it'll be motherboard video (which doesn't necessarily rule out the possibility of an upgrade)
once you can see the PCI slots, you'll have to find out whether you have
an AGP, PCI-E, or plain PCI slot somewhere; PCI-E is pretty much the only one that'll have recent cards -- AGP pretty much stopped a few generations ago, and if you only have regular PCI... you might as well forget about it
there's more stuff to take into consideration, such as your PSU wattage, driver compatibilities, and even whether or not the card will even fit in your case (which is unfortunately trial-and-error sometimes)
This post has been edited by Xgoff on Jun 28 2008, 01:21 PM