Hello and welcome to the forums
As gkay said P55 supports SLI/CF at dual 8x mode with the exception of those 2 motherboards.
However if you use single card on a P55 board,it will operate at 16x speed.
Just to be clear, I only plan to use ONE card, period. My case design is kind of stupid, and the GPU runs into my hard drive bays in the top slot. So, I just want to move it to the 2nd one. If I have to use the top one I still can, but right now I have a rather peculiar rig keeping the 2nd hard drive out of the way.