You can put a graphics card into that slot and expect it to work but why can't you simply put it in the blue 8x slot?
The slot is mostly used for RAID controllers that need a lot of bandwidth but not as much as graphics demand.
A graphics card can actually work all the way down to PCI-E x1 mode but thats is not recommended.
Your motherboard won't do x16/x8. The x16 slot would only do x8 as well so it would be x8/x8 because the P55 chipset unlike the x58 chipset has limited bandwidth. This is not saying that x8/x8 is no good, the difference is only a few %, not noticable until you get into tri or quad fire.