Well, now that the 7800GTX is out and can be had for $500, it does negate the idea of building a 6800GT SLI setup, because that would be more expensive and performs worse overall, not to mention the power and heat issues. But if you already have a\one 6800GT and a SLI mobo, it might make sense to get another 6800GT for SLI, as it would be cheaper then upgrading to 7800 GTX (depending how much you can sell the 6800GT for). Before 7800 line was introduced, 6800GT SLI was in fact a good option for the extreme high end - performance was very close to 6800U SLI and it was cheaper. But now it is definetly lost its value. SLI is always good value only for the extreme high - using 2 highest end cards together.
Worst SLI in history = 2x6800NU some people paid 2x$350 for it just a couple of months before a single X800XL was available for $250 with MUCH better performance.