JKay, if you're not planning to do this build for 2-3 months from now, you should probably just keep an eye on the space race between ATI and Nvidia. By the time you're ready to buy, Nvidia may have leapfrogged ATI and maybe ATI will have leapfrogged them and so on!
It looks to me as if all of the GTX 280 cards are double-wides, as well, at least those that are air-cooled. Amazingly enough, though the 4870 X2 was only announced a few days ago, I found this announcement of a company planning to release a water cooler for it:
As usual, one of the benefits of all of this competition is driving prices down and features up. Since the 4870 X2 was announced by ATI, Nvidia has dropped the price of the GTX 280. Older cards from both manufacturers are coming down in price. This is probably a good time to pick up a Radeon 3870 or GeForce 9800 series card.