XFX 7870 Troubles, woe is me!

Hi guys, I'm a bit new here, but please hear me out

I recently rammed a 7870 into my mediocre computer, finally replacing my old 4890, and blindly expecting the shiny new component to work first try. As you might have guessed however, the card didn't work. Well, it seemed to do everything right, I heard output, but no video input was to be found. I assumed immediately that this was the PSU, since mine was a generic 720w~ thing from Maplins. So, I invested in an 80+ rated Silverstone PSU, being absolutely positive this would solve my problem. Even then though, I had the same result. Giving up, I took my computer to a computer repair shop, where they told me my motherboard (a 1155 socket P8H67-M LE) was my cause for concern, since apparantly, after his testing and rebuilding, the boards PCI-E 2.0 slot was incapable of running the card. While the man sounded pretty certain, after some research, I have discovered this should'nt actually be a concern, since PCI-E is backwards compatible. Can someone help me? Is buying a new motherboard really the best option for me?

It should be fine with the Pcie 2.0

go into your device manager in windows after re installing your old card and disable the old one. then power off your computer insert the new card boot into windows and  install the new AMD drivers 

>It should be fine with the Pcie 2.0

>go into your device manager in windows after re installing your old card and disable the old one. then power off your computer insert the new card boot into windows and install the new AMD drivers

Just tried this, no luck :(

try something simple like is the pcie 6 pin cables plugged in fully or is it in the right slot