You can't combine an Nvidia card with an ATI card. 2 different companies, 2 different architectures, 2 different pieces of hardware.
I'm not sure about the 7900gt, although i'm pretty sure it doesn't support sli. Anyways, thats a mute point since it would terribly reduce performance even if it could.
When you crossfire/sli two different levels of cards, say an ATI Radeon HD 5770 and a 5750, the 5770 has to be scaled down to the performance level of the 5750 to work.