I have two questions. First, if a motherboard only supports PCI, is there any way to get an AGP card to work with it? I doubt it, but just incase. Second, how much better are AGP cards than PCI? Is the difference humongous?
In answer to your first question. No. No no no. Second, AGP cards are much, much better than PCI mainly cause it gives you a dedicated path between the slot and your CPU. It's also a lot faster because of the way AGP handles things. Textures, for example. A PCI card would have to copy a texture from RAM into your card's buffer, whereas AGP cards can read textures directly from RAM. It's just generally a hell of a lot faster. There isn't a single serious gaming card isn't either AGP or PCI-Express. Talking of PCI-Express, that's even better than AGP. Confusing you now? Good. PCI-Express is a big advancement upon PCI, it's not very widespread at the moment though. See AGP 8x is capable of a maximum theoretical data rate of around 2GB/s. PCI-Express however is theoretially capable of around twice that. So to sum it up in a sentance. AGP is better than PCI, but not as good as PCI-Express (in theory).
If you have an AGP card and are looking to upgrade in th future, you should look into the motherboard I have, the ASROCK Dual Socket 939 (Assuming you have a socket 939 AMD processor, which isnt much of a problem due to the recent massive AMD price drops) It has AGP and PCI-E slots. Quite useful.