Future games can't be predicted that easy, the 4870 X2 would be the fastest cheap solution up there. You'll basically be getting 2 4870s compressed in 1 card that has higher ram space, and costs a little more.
Save space, save money in the long run, and have the option for quad.
but personally for that resolution I'd go with the 4870.
What motherboard do you have, because that could impact if you want to go Nvidia or ATI.
yes but pretty much overckill for the resolution ur he's running. Even a 800 GT can put up a fight at that resolution.
I'd say buy a 4870 and if the need arises, buy another 4870 or 4870 X and CF them, no need to rush into buying if no games will use that amoutn of power. Wait till the new Crysis game comes out, it will be more demanding than Far cry or any other game coming this year (from what I read in reviews) see how it runs on 1 4870, if you need more power than grab another card
I'd be a much better idea to get a single 4870 and play it by ear. The next gen of games will bring a next gen of cards. Why spend $600 today when possibly you can spend $300 now and another $300 a year or so from now. Especially with the ATIs you could even crossfire across different generations though that's very much ymmv.