depends of the budget.
But I had once 2x ati4970 and I regreted it.
Microstuters/driver problems/1 card that was better than the other....
If you're lucky you won't have any problem, but if not then it's a real pain :(
Personnally I would save to buy a bigger card (dont take a card like the 6990, wich is 2 card in Xfire on physically 1 PCB)