It's fine if your are running at resolutions under 1920x1200. The x8 is still PCI-E 2.0, so that mitigates the effects somewhat. There are only a few frames per second difference at 1680x1050 in most games, when you compare the x48 to the P45.
ALL P45 boards that have dual PCI-E run at x8 in crossfire mode.
If you were going to go that route I wouldn't get a DDR3 board. The memory is more expensive and there is no performance gain.
A P5Q Pro or -E will save you cash and deliver the same performance. If you have a very large monitor or need bragging rights, get an x48.