Sigh.. i guess you get what you pay for some days when you ask for free advice...
Don't listen to Hundredisland, he is either joking (I sure hope so) or currently impared..
2 4850's will run mostly anything and perform better than any single GPU. It is not currently worth investing in a more powerful single GPU given how much more that would cost you than just one 4850 specially cosidering the performance of 4850's in corssfire. Crossfire is not free of errors though, by the time suport for 4000 series crossfire starts to drop off you will be able to get a far more powerful single gpu card though...