Hi angel,
AGP speeds weren't that critical to be honest.
You could have a 8x card in a 4x slot and the card would just run a bit slower. There should be no other compatability issues you will see or experience
In regards to AGP and PCI...they're completely different interfaces and will not work together. PCI goes in PCI slot, AGP goes in AGP slot and PCI-express (your computer doesn't have this) goes in PCIe slot...PCI-X is server specific and shouldn't be bought for desktop computers
