Some online store told me that I can't mix these two brands in a Macpro. If the machine came with Ati Radeon then I should put the same brand on the second slot. How true is this?
That's true, you can use either one ATI Graphics card, one high-end Geforce card, or multiple mid-end Geforce cards. That last option is only used to connect many displays to the system.
Thanks for clarifying that. I wonder what's gonna happen if a mix was done. Will the cards be totally useless?
I guess it's gonna make a magic smoke effect. Apple hasn't really explained this. The guy from this online store just said "Apple just doesn't recommend it. And if you want to customize is that way, it will void your warranty if anything happens." Thanks dude!
You're welcome It has never been tried by anyone, because no one would buy two expensive high-end graphics cards to try blowing up those and their expensive Mac Pro machine.