I agree. There is no real advantage to buying two 6600GT's when you could just buy one 6800GT. Furthermore, I totally agree with you, that the upgrade potential is not that attractive, because buy having two of a given card, you still aren't getting the latest advances in the hardware.
It's not all about speed.
That's what is funny about those proporting the buy one now, then buy another when the price is about half. It's a dumb idea thinking that, unless you plan to play the same games you are playing now, but then why would you need the extra power of 2 GPU's? The only reason SLI is popular (or why it should be) is the fact of buying two of the top-of-the-line GPU's, right now. That gives you better performance than is possible with one video card.
I have to admit though, $200 for a board that has as much as the A8N-SLI Premium does, you are getting a lot of bang for the buck, especially with the fanless design.