I would say the 4870 X2 would be enough for you at that resolutions (though I wouldn't place my bets on Crysis, at least for it to be smooth). But if you want raw power the 280 GTX sli is your best bet.
280 GTX sli > 4870 X2 > 280 GTX
Alone the 4870 X2 is up to 30% faster than the 280 GTX but is around 200$ (depending where your from) more than 1 280 GTX. If you have an sli board, the 280 GTX should be ur card.
For me its very re assuring that if sli/Crossfire isn't support I have a 280 GTX single GPU under the hood. I would say that the 30% difference for dual vs single is very impressive for the 280 GTX.
But this all comes down to either money or performance, sometimes both
4870 X2 in general would be my number 1 pic for price/performance
2nd choice would b the 280 GTX since it still has enough power under the hood for that resolution (not quite as much as the 4870 X2, though you might not think its worth it)
Last would be the 280 GTX sli (no need to jump in so quick unless your an enthusiast.
Though if you have an SLI motherboard, I'll say it again the 280 GTX plays more in ur favor. I just can't say no to the 4870 X2
just wicked over all card.