It would be pretty hard to mix them up. Most of the cards out there are AGP, you have to look a little harder to find the PCI ones. The 5500 is one of only a few cards out there still made for PCI. Thats why I asked him which one he had. Whenever I see 5500 I suspect PCI, and there are a few problems unique to PCI slot cards.
Besides the data interrupt issues and the slower bus speed, PCI slots have a couple other things against them.
1. Because AGP is a dedicated slot, even people with onboard video often upgrade to an actual video card fairly quickly. Once in, the card will probably stay there as long as the computer lives and is in use (or at least A card will be there). PCI slots, OTOH, might sit empty and unused for months or years before someone needs the extra slot for something. Dust can accumulate inside the slot, and muck up the contacts when a card is inserted. Also, moisture in the air can cause corrosion on the metal contacts inside the slot, making good contact with a card less likely.
2. AGP slots also have dedicated power from the PS. Whether its 3.3V or 1.5V, they get constant power (which you really want in a video card, especially with modern games). PCI slots share. To make matters worse, the problems above that affect contact between the card and the slot not only affect its performance, but affect its power draw too. A modem or a sound card probably doesnt need much juice, but a video card NEEDS power. Power it wont get as reliably in a PCI configuration as it will in AGP. New cards have a direct line to the PS, elminating that problem (as long as the PS can keep up that is).
