Open up the side of your case and above the PCI slots where you have your modem, sound card (if not attached to motherboard), and anything else.

Check if there is a slightly larger usually black vertical connector separated from the PCI slots usually above. That is AGP.

If you do have AGP, you can get every single video card, if you don't, you have to look for a PCI video card

If your buying a new video card, first look around for a good one, then check www.pricewatch.com (If you don't live in the US, I think theres pricewatch for other countries) for how much you can find that card. My new 5600 was $220 at pcclub, and it was on pricewatch for $123. You can always get things ultra cheap. You might also want to check a site like http://resellerratings.com/ so you wont ever get ripped off.

just my 2cents