A few things you need to know first:
For either GPU, PCIE 2 provides enough bandwidth.
Most current generation GPU's don't take advantage of PCIE3.
What is your current budget for a graphics card? As well as your complete system specifications? CPU, RAM, SSD, HDD, Power Supply name/model and wattage. Also what resolution do you use? What kind of frame rates are you expecting/wanting in games?
Your motherboard is PCIE 2 by the way.http://www.techpowerup.com/reviews/AMD/HD_7850_HD_7870/13.html
You cal also download GPU Z to show the details about your GPU:http://www.techpowerup.com/gpuz/