I would go with one 7900GT. 2x 7600GTs bench pretty close to 1x 7900GT. The price is about the same either way. Since the price and performance are really comparable I guess it comes down to heat and power consumption. So, you might as well get a 7900GT.
i basically agree with elbatrop. in some cases the 7600gt's may be better though. they would probably do quite well at high resolutions and high levels of AA or AF
I would get the 7900GT because later on down the road if you ever want to upgrade you can upgrade into another 7900GT for SLI, however, if you get 2 7600GTs you get to replace both of them.