I've been looking at this, too - I've decided to go for just one 6800 Ultra card rather than the SLI set up as it seems my chosen monitor (an Apple 30") won't run at its full resolution with two cards - it only supports the one. And apparently the drivers are a bit flaky and not many games support SLI.
However, i think I will ask Alienware (I'm pretty sure that's the box-shifter I'm going for) if they will build the PC with a slot for a second card so that I can upgrade later if the drivers catch up ... or perhaps put in two yet-to-be developed cards in a year or so's time.
I rather agree with you about the point of buying two 6600s when they don't quite match the performance of one 6800.
Let us know how you get on!