3 interface standards for video currenyly exsist...
PCI, 133 mhz max transfer speed, oldest, slowest.
AGP, in 2x, 4x and 8x. (x=PCI Bus speed) On its way out.
PCIx, or PCI Expess. The latest and greatest. 16x.
What it all comes down to is how fast can data be moved to the video card. Todays game are so graphicially intensive that the major bottleneck in performance was the cards interface. With SLI and CrossFire the latency has been reduced to a minimum.
When your sitting there waiting for a game like Quake 4 to "load", what happening is the texture data is being decompressed from the harddrive and loaded into your graphic cards RAM, (of course theres data being loaded into the system RAM too). As one plays the game as the screen changes data moves back and forth, the more "bandwidth" or speed the better.