Not possible, especially at that resolution as you would need 2 4890s to get decent FPS at medium to high detail and they cost about 200 each, i think your monitor cost more than the rest of the build, save up a bit if you want decent frame rates and then check back, i dont think you can get decent FPS out of anything costing less than about 1000 at that resolution.
Yes, the 4850 may be faster, but the OP wants eventual Crossfire, and two 4850s will need a budget-busting PSU.
Newegg is also compelled to charge extortion to people living in Tennessee.
Shortstuff & Dirtmountain have posted excellent "almost" builds; they'd both be okay, then you'd kick yourself for not waiting until you could spend a little more. If you can't, then either will be decent. If it were me though, I'd grit my teeth and save up another $200. Then, I'd get an AM3 mobo with DDR3-1333 RAM, the X4 945, one HD4870 to start, and a 750W Truepower New, which would allow for Crossfiring with a second HD4870 later. With those changes, I think you'd need to spend closer to $750, but you'd have much more satisfying long term results. An alternative would be to get an AM3 790GX mobo and use the IGP for surfing and movies, then add the video card(s) for games as your budget allows.
You should expect your computer to cost more than the monitor you intend to have it play games on. I cannot in good faith suggest spending only 600 on a computer for that resolution monitor as you should really have a crossfire capable board and a good 750 watt PSU which has already eaten up most of the budget, its stupid to get a monitor that size just to be forced to play on low detail settings in order to get 30 FPS because you can no longer afford a good computer to go with it. Save up for another month of two then come back.
Wait for i5, it can only make prices drop or stay the same and in doing so you will wait for the HD5xxx series which will certainly make graphics card prices drop. waiting 2 weeks could prove very very beneficial in this case. just my 2 cents