RhinO, you make excellent points regarding the decision to build immediately rather than wait for the next wave of "latest and greatest".
However, I would urge that in this case, you do wait. I know. You're like me and you want that system purrin' away on your desktop now! But, you're unfortunately steering your bright n' shiny new Mazda RX-8 down a dead end alley and the turn-around space is mighty tight, if you take my meaning.
It's not so much an issue of SLI or even nForce4, but the upgradeability of the whole PCI Express vein. True. There really aren't much performance gains at the moment, but who's to say that some time next Summer, ATi won't release their next beast that just smokes anything that's out right now? You'd be golden if all you had to do was pluck out your "current" (I'm talking in future tense, of course) 6800Ultra and dropping in that incredible new card in the shrink-wrapped box that's still wet with the sweat of your eager fingers.
If you choose to go with the currently-available hardware (which is excellent nonetheless), you'll not only have to replace the video card, but the motherboard too as well as do all the painful tweaking associated with that venture.
This is not to say you have to wait until next Summer, by any means. Actually, my rubber ball (the crystal's in the shop) tells me that by mid-January, we'll be in good shape. Don't let me stop you, however. I, personally, would indeed just wait, literally, a few more weeks.