How to SLI a system need expert guide
SLI will be better than 1 VGA and 1 for physX. Unless you specifically play a bunch of physX games and a weak CPU, you'll find a better benefit to having the additional rendering power.
Yes.
Faster card will downclock to the speed of the slower card. It will effectively run without the OC.
I don't know about memory bandwidth differences; I think that should work, since they are both the same size memory. If they are not the same size memory, then the card with extra memory has the extra memory disabled in SLI.
Yes, it'll work fine.