Obvious, there is a cost factor. A single 7800GTX card will out perform 2x6800 Ultra in SLi mode. That tells a whole lot. The 6800GT was the "one" to get back in the days before the 7800GTX came along. They're nice cards and if money is an issue, the 6800GT is a good buy. An alternative to the 6800GT would be this one:
SAPPHIRE 100101SR-RD Radeon X850XT PE 256MB 256-bit GDDR3 VIVO PCI Express x16 Video Card
or something similar depending on who you want to make it. You'll hear a lot of ATi's new R520. It's safe to say that the R520 is more of a competition to nvidia's 7800GTX so we're dealing with some high end stuff that a lot of people can't afford.
The 6800GT is what you need to get or if you want to pay just a little more, you can look into the X850XT PE.