The difference between 2x 8x PCI-E 1.1 and 2x 16x PCI-E 2.0 was zilch, zero, nothing.

There was a review somewhere comparing an MSI P35 with 16x/4x, Asus P35 with 8x/8x, and an X48 with 16x/16x and there was no difference between the last two. I only got an X48 due to impatience, but graphics performance on the P45 shouldnt be any different to X48. The only difference will be the price tag, and overclock ability.