Now curious, make sure you don't get confused. Preformance wise it goes like this for vidcards:
PCI-Express > AGP > PCI
And btw Chris you forgot to mention:
PCIE supports ATI Crossfire
Btw, ATI Crossfire > SLI.
Main advantage cause it works good in any game, even ones that don't support dual vidcard modes natively. This is where SLI fails. If a game doesn't support SLI, it has virtually no framerate gain over a single vidcard, if anything a loss.
note, COR doesn't support SLI rendering modes, thus the framerates.. yeah, so I find SLI a waste of cash until nvidia does something about this. I mean, they bought out 3DFX, and they had solutions for this that they used in their voodoo2 lineup which (for now) was the most effective dual vidcard setup. ATI did some good solutions for games that don't support dual-vidcards with Crossfire.. I don't see why nvidia dropped those solutions.. but now they're paying badly for it.
Btw, good readup on ATI's Crossfire:
The dual rendering mode of SuperAA sounds freakin' sweet. So like better visuals inna dual vidcard solution with minimal framerate loss (if any). So it won't be doing like 150F/S.. who cares? 14xAA with a steady framerate? that would be an incredible sight...