In an earlier post, it was mistakenly written that all 4 GPUs were contained on one PCB. It is in fact two video cards—two GeForce 7800 dual cards strapped to each other like a daughter card via 2 SLI bridges, so the card itself will occupy two PCI Express slots. If it still isn't clear, check out the pictures for yourselves here, here, and here. That sure takes dustbuster envy to a whole new level.
Here is a slide that shows "super linear speedup" benchmark numbers in F.E.A.R., Doom 3, and Quake 4 comparing a Quad SLI system to a single GPU system although it is difficult to make out the actual scores.