i have a mercury pig31t motherboard what kind of graphics card can i install in it

it has PCIE slot, so any PCIE 3.0 or 2.0 card can be used in it. the limitation will be more towards your CPU and power supply. you could put a TITAN X or a lowly GT 710, or anything in between as long as your power supply can power it. NOTE: the Titan X is a really bad idea, the bottle neck would be terrible.