Nope. I can't max Just Cause 2 @ 2560 x 1440 with my 5970. You'd be surprised.
I'd think the sweet spot would be two in SLI, best price/performance ratio, but hell, if you have the money, why not throw in another.
Though the 5970 might have 4GB versus 1.5, it might have Xfire problems.
I don't know Just Cause 2 nor played it so the Xfire was just a assumption on my end.