1) Running a single card will have no impact on performance with any motherboard with USB 3.0 & SATA III. It will run at x16. You won't see issues, if at all, until you run dual cards.
2) The motherboard I recommend the most for P55 is the ASUS P7P55D-E Pro
but since dual cards aren't in your picture, it isn't necessary. With that said, I would go with the ASUS P7P55D-E LX