Karmafree,
You can only upgrade to an AGP video card if you have a AGP expansion slot on your motherboard, otherwise you will need to get a PCI card and just replace your current one. You can consult your motherboard manual to see if you have an AGP slot. If necessiary, you can remove the case cover on your computer and look at your motherboard to determine if you have an AGP slot, it will look similar to PCI(white slot) and ISA(black slot) slots except AGP slots are brown and are usually located close to the processor. Make sure you unplug your computer and take the proper ESD precautions.