PCI- forget it, if you only got a pci slot, then get a new computer
AGP- performs the same as PCI-e, it's just older
PCI-e- performs the same as AGP, even though it has twice the bandwidth, since games don't even use up to AGP's bandwidth
PCI-e is the way to go now, since it's newer technology
diabloII is right, if not AGP or PCI-E you are just screwed
AGP is fine for now, if getting a new motherboard get PCI-E since AGP will be phased out eventually