You have three choices for crossfire currently:
P45 - works for 4850s in CF if you are only using a 22" or smaller monitor, but generally limited in bandwidth.
x38 - Full bandwidth crossfire and not much different than...
x48 - Considered faster than x38, it's basically the same chipset, just ramped up a bit. Because it's considered better, manufacturers tend to put more bells and whistles on x48 boards.