As far as I know, in order to SLI you have to use 2 cards that are from the same manufacturer with the same exact everything.
You will not be able to use a XFX 7900GT with a BFG 7900GT in SLI.
Just use the one card, Ive used a 7900GS which is essentially the same, and it gets superb performance @1600x1200.
just stick w the one card
You can SLI different brands. It doesn't have to have everything identical, just same GPU.
Any 7900GT with any 7900GT for example.