This build leaves you the option of crossfire, if you do not plan on going that route, you can save about 40 bucks by getting a smaller PSU (Corsair 650TX), and the next motherboard down (UD3P).
I think that the $650 sbm build with 2 hd 4850 would beat the proposed build in frame rates in games (depending on the resolution). I think a better option might be going with the smaller psu and a none crossfire p55 and dumping the $70-80 you saved into a better graphics card like the hd 5850 which is for $260. Of course you wont be able to simply crossfire in the future.
I think you will have to make too many compromises to squeeze a HD5850 into this build, here is something you could do, building on Hamit's suggestions;
So the difference stays the same, you lose the option of crossfire, but gain more performance from a better GPU. If you ever plan on gaming at 19x12 or higher, then I would stick with the initial build and go with crossfire, if not, then a single card solution will be plenty adequate.
Gaming is a low priority so its hard to recommend such a powerful GPU as the 5850 or crossfiring multiple GPUs. New systems that you want to last really should have DX11 so its hard not to recommend the 5850. Really this build seems like a good candidate for a 5700 series GPU that should come out in the next month or two, but the timing is to early.
Another option is to go with a relatively cheap card now, like a 4850 and then upgrade it to a DX11 card in a couple years when DX11 becomes more prevalent.