While there are a few differences between the two, we only cared about one of them. We wanted to know the performance difference between the X48 and the P45 when running Crossfire. While both boards carry two physical x16 slots, when two cards are installed the electronics behind the slots on the P45 tune back to x8 on both slots. The X48 on the other hand continues to run both slots at x16 independently.
Check it out at TweakTown.