I have a motherboard with an AGP and PCI-e, if you have both being used, the same brand & make, it is supposed to improve over the performance over having one..
SLIing two different GPUs will make it not to post. You need exact GPUs on each. I don't think the brand would matter though. Correct me if I am wrong.
u are right. Needs to be the same vid card (like a 7600GT with a 7600GT will work, a 7600GT with a 7600GS doesn't work) The brand doesn't matter anymore, there was a change in the drivers a while ago that allowed the use of different manufacturers vid cards in SLI.