You don't need 4850 xfire. I play tf2 maxed out with 1920x1200 and your on a 22" so you will be fine for now.
I suggest you buy a single 4850 now, and you can xfire later if you feel like it.
Performance increase will be like going from a 8800 gts to a 9800gtx+ OC'd for one 4850. Xfire will be like going from an 8800gts to above a gtx280 when the game takes advantage of both cards.