While twin 4850s will give you the most graphics power for $200 today, I dont like them from a power, heat and longevity standpoint.
They dont have DX11, which I am assuming will become important in 1-2 years. Your estimate may be different.
For $160 you can get a single 5770 which takes far less power and only one PCIE slot so you can get a cheaper motherboard and power supply. If you do get a p55 motherboard that will x8/x8 crossfire, even then twin 5770 will take less power than twin 4850s while stomping them in performance.
Now thats is HORRIBLE advice. ATI never specced a 4850x2, its a unique contraption that Sapphire kludged together. They are not officially supported, have always had glitchy problems and are no longer produced.
For motherboards I tend to stick with Gigabyte, ASUS, DFI and EVGA as I know all those companies have fairly good track records for reliablilty and support of gaming motherboards.