If you know what you're doing...it can be a very insightful decision. For example...

I was initially going to buy a 7900GTX and then maybe a year or two down the road, SLI it. Problem with the GTX is high power consumption, it's very large and generate a good amount of heat. What I am now doing is getting the 7900GT. Two of those in SLI outperform one GTX. For me, it was more of a benefit to take the slightly less uber card......and buy 2 of them.

Less hassle and it isn't that much more than just buying 1 GTX.