Hey, and first, welcome
Could you possibly tell me what motherboard you have?
It should be written on it. The model number would do fine.
Its not very common for you to find AGP slots and PCI-E slots on the same one you see. Only a couple of boards I know of even bothered doing this, for people who might want to upgrade to PCI-E later, but still had an AGP card.
If you have a PCI-E slot, this is the way to go, as its the newest form of communication between the card and the motherboard, which is a lot faster than AGP, which it replaced.
PCI-E 16x is the slot you want for video card useage, as it is the fastest available at the moment, and if yours has one, you are ready to go.
Standard PCI sockets are still used for expansion cards, like sound, internet LAN cards, etc, etc, but I see that over time, support for PCI-E on many of these cards, will replace PCI completly.
Info on PCI-E