The motherboard supports PCIe 2.0, the video card is PCIe 3.0 and will claim backward compatibility.
The motherboard was made 8 years ago and did not know what the future held. Intel® Desktop Board DP55SB Product Specifications
I would suggest a firmware update to the video card, if one is available that addresses PCIe 2.0 compatibility.
Otherwise, look for a card that is truly backward compatible.
My personal opinion is, if you have a nice, new graphics card, get a nice, new motherboard. Otherwise, you will be operating in a degraded mode with PCIe 2.0.