cant a p45 mobo use crossfirethe x58 is socket 1366 only so that has no need to be mentioned here
a p45 is perfect if no crossfire is needed
P45 I think is 16x & 8x when in crossfire mode.
is there a major performance loss in 8x + 8x from 16x + 16x?
Depends on the board i think, a certain p45 board is not far behind the x48 cant remember which though maybe the maximus formula ?
is there a major performance loss in 8x + 8x from 16x + 16x?