Two 7600 GTs cost more money than a single 7900 GTO.
Two 7600 GTs need more power than a single 7900 GTO.
Two 7600 GTs are twice as prone to failure as a 7900 GTO.
Two 7600 GTs are not as fast a a single 7900 GTO.
If you are willing to spend a few minutes, check the forum for all those senseless SLI threads. People just keep asking whether to SLI two 6800 or get a single 7900 or to crossfire two x1600 instead of getting a sinlge x1800,,,
The replies are all the same.