PCI Express x 16 will become the standard a few years from now. I am going to stick with my AGP Pro (on the i875 Canterwood mobo)for now, because no programs can take advantage of the speed that PCI-X works at.
SLI was really great when you hooked two VooDoo 2 cards together, but with todays cards operating as fast as they do, will you actually be able to see any difference? The X800XT PE works at 1.1 GHz, where most peoples cards work at half that or less. The 6800 Ultra OC works at 1.2 GHz. How much more do you need? Unless you are a benchmark fanatic, nothing.
Alienware also is coming out with its version of SLI. That system will set you back $4K. Ouch.