HIS 7870 causes PC to not boot

Hi, I recently bought a HIS 7870 2GB to upgrade from my 6850 from newegg. I received it and installed it in my pc. Upon booting, there were no beeps, no display on screen. It was weird. I switched back to my 6850 and it all worked well. I thought it was the video card that was defective, so I RMA'd it. I just got the new one tonight...same problem. I just don't get it...it doesn't make any sense at all. I have a 850W PSU, never gave me problems. I have an AMD FX-8120 CPU and a ASRock 970 Extreme4 mobo. I don't see where the problem lies... Can anyone help me?

Thanks a lot