Well Usually PCI was The older Counter Part as a Expansion Slots in PC And AGP was considered for extreme Graphics and eventually AGP Overtook PCI But now after the release of The PCI Express TEchnology I suppose it will give a way better competition to The AGP 8x Both In terms of speed And performance 8) .....But just one thing Ur motherboard Should be able to Support the PCI-Xpress Interface which is very less in normal motherboards and AGP is a Common Input.....But well The Future Is never Set so just sit back and enjoy what the technology offers....Try This links for more technical Explanation......
*www.sharpened.net/helpcenter/answer.php?57
*www.cs.umd.edu/class/fall2001/cmsc411/projects/agp/pci_vs_agp.htm
and lots more If u Can google it mate....