I did exactly what you're planning on doing. I have a 6600GT and an ASUS A8N-SLI deluxe, and L8er on I plan to add another 6600GT.
You NEVER get much back when selling computer components, so it's cheaper to get a 6600GT now and another L8er, rather than getting a 6600GT now and later replacing it with a 6800GT.
And performance of SLI 6600GT's is around the same as a single 6800GT.
RoD