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No............which ever card runs the lowest will bring the other card down to its speed.
An i don't think you can run 2 different models like 6x with a 5x or 7x series im sure they must be the same series of cards although the brand doesn't matter
Yes they would have the same performance at the same vram and core clocks, they are the same thing.
No, you can't SLI them
Also, SLIing 6xx/7xx cards at different clock speeds doesn't change the speed of either, they are independent
It has to be the same chip series and vram amount. a 2gb and 4gb wont work
Theres a thread knocking around here some where from the last 2-3 days clearly explaining that the cards will not run at the same speeds an that they will be bottlenecked by the lower card.
Il have a look around for it an post or pm you the link.