If the north bridge on a motherboard is made by Nvidia does it have to have a Nvidia graphics card for it? And if not is there any performance advantage in pairing an Nvidia card with an Nvidia motherboard? I'm wondering because I prefer Radeon GPUs and I was wanting to build a system with a specific motherboard but it has a Nvidia north bridge and is an SLI board.

Shouldn't make any difference, all the northbridge chip does is conncet the CPU to the memory.