I'm assuming you want an extreme-performance gaming rig.
Get Quad-SLI if you are also buying that 30" LCD. Or if you think you can really use those nifty SLI-AA modes at a lower res (like 1920x1200).
Anything less than that and Quad-SLI actually performs slower (in some benches) than regular SLI.
May I suggest the E6700 and a single 7950GX2? It would be mighty hard for you to be disappointed with what this combination can give you in terms of performance.
$380 to go from a single 7950GX2 to dual-7950GX2 does sound like a deal, if you can actually utilise the power with a 24"+ display and like SLI-AA.
$186 to go from 2GB to 4GB also sounds pretty good. Will come in handy for a future OS upgrade, I think. But for now, you prob don't need it.
$310 to go from an E6700 to an X6800 doesn't sound that good a deal for the benefits.
Just my humble opinion.