|Nvidia all agree that the faster bus will give better bandwidth and scalability, allow large request size and pipeline depths, and give isochronous support. The bandwidth will deliver simultaneous 8GB/s concurrent peak bandwidth in X16 mode. PCI Express will also provide a pipeline depth of up to 256.
Graphics cards using PCI Express tech will, however, require 75 watts max, and that means a few things. First, the 12 volts on an ATX power supply won't do the trick. The answer will be to use a 2x12 connector with the same pinouts as server SSIs, which requires a 300W power supply.
The other problem is that machines will need to use side panel vents, ducting, and use large fans.
Not only CPUs will need more extreme cooling solutions in the future, it seems that graphics cards will need them to..
Some more information about differences between managing non-local memory in AGP and PCI Express can be found at The Inquirer