I wouldn't run 2 of the 4870 512 cards in crossfire, if you have the resolution to really take advantage of 2x 4870 cores then your going to find the 512mb of ram holding you back. Hard OCP has already shown that the extra 512mb on the 1gig cards is useful in todays games with a single card configuration. I wouldn't be surprised of a single 4870 1gig card actually out performs 2x 4870 512mb cards in some cases.
The first question is does your motherboard even support Crossfire or SLI? Even if it does support Crossfire I would start out with one 4870 1GB and then add a second if you want more performance.
Your board supports SLI only, not X-fire. Now the new x58 intel based boards can do both SLI and X-fire. But that's a whole other situation. Back to your current situation, at that resolution grab a dirt cheap 260gtx or HD4870 1gb, your choice...