Really nobody is going to know for sure yet. It was rumored that ATI has some kind of shared/distributed memory controller scheme that could theoretically help performance moreso than two cards in CF. Even without this, the interconnect will be more local so there should be less latency between the cards. Assuming they can keep the clock speeds the same, the 4870X2 could come out very slightly on top with a bigger gain if the rumors about the MC turn out tu be true. We won't really know until the NDA is lifted on the architecture details, or maybe not even until the 4870X2 launch.
Regardless, at such a low resolution, I think more than a single 4870 is overkill, and in my opinion, any dualcard/multiGPU solution is a bad idea for "future proofing" but this is going to boil down to personal preference. If that's the road you want to take, the best option would be to buy a single HD4870 and then wait to buy the second one until the price is much lower. I would be surprised to see CF yield any benefit at that resolution though until much farther down the line.
If it comes down to a contest between the HD4870 and the GTX280, an important thing to keep in mind for future proofing is that the HD4k series is showing a very small loss when enabling AA. So while the GTX280 may give higher framerates now, the 4870 could let you game at higher settings for a longer period of time game-development wise. I'm not sure to what extent this will be relevant at your resolution, however. I dare say that I think the HD4870 is a vastly more cost-effective solution because for half the price both cards are going to give you more than playable framerates.