I don't see any reason to forgo a 7800 GTX for 2 6800 ultras, it doesnt make sense. First off, it would cost more money. Secondly, one 7800GTX is incredibly powerful, the most powerful card on the market in fact. It runs toe to toe with 2 6800 ultras running in SLI. And finally, if you buy one 7800GTX, you can simply get another later and run the two of them in SLI mode instead of buying a whole new card for more money.
I really can't see any reasons to justify the purchase of 2 6800 ultras over 1 7800GTX.