IMHO SLI with a 7800 is pure bragging rights and nothing more. The 7800 solo performs insanely good enough to run all the current games in full detail with all the eye candy turned on at 1600x1200 and still has enough power to run AA...who really needs more than that?
With games designed to take advantage of the Phys X chip running a single 7800 with the add in card will blow the doors off running an SLI rig without one simply because the CPU won't be a bottleneck any longer. Where's the sense in stuffing $1000 in video cards into a case just to stroke your ePenis when one card and a $300 PPU (Physics Processing Unit) will allow better gameplay on games coded for the PPU than the SLI rig will give you without it?
Besides, since there are boards with PCI-E slots in between the 16x PCI-E slots AND the 7800 is a single slot setup you could still stuff a sound card in between the graphics cards if you add some cooling to move air into the top card (example, the ASUS SLI board has 2 PCI-E ports, use the one closest to the bottom card and you've still got some room for the top one to breathe) and that would leave you all the slots below the bottom vid card to fill with other goodies.
I've run systems with every last PCI slot and my AGP port full, it can be done.
BTW, Rob, good read.
Phenom II 940 Black Edition quad core @ 3.5Ghz
2x1 gig OCZ PC26400 Platinum, 2x1gig GSkill PC26400
Some other stuff,
Even more stuff,
If the automobile had followed the same development cycle as the computer, a Rolls-Royce would today cost $100, get a million miles per gallon, and explode once a year, killing everyone inside. --Robert X. Cringely, InfoWorld magazine