I'd go with the 2 6800GTs. While neither is quite as fast as the 7800GT, the two of them combined should run circles around a single 7800GT. I would expect them to be able to give acceptable performance for a longer period of time than a single 7800GT as well, so ultimately giving you a little more life out of the system for the money.

OTOH, you could always go with just the single 7800GT today, and then in a few months when the prices are significantly lower, add in a second one. I can't claim to know anything about setting up an SLI configuration from a single card and how easy or hard it will be, so if this seems like an attractive idea, I'd look into that aspect of it.