ATI-GPU and Nforce 4 MB hows is the performance as opposed to and ATI based GPU and an ATI based motherboard. Nforce 4 and Nvidia cards do they work together better than ATI and Nforce 4 MB.
There shouldn't be a significant difference. Technically speaking, they can't make a motherboard favour a certain videocard. Unless it's a specific set-up, such as SLI and Crossfire, those aren't backwards compatible.