My gpu won't display anything

[nicbread]

Hi, I've got my old i5 750 PC, and I bought a 7870 for $40 off gumtree (Australian Craigslist). I recently put them together to make myself a Gaming PC, I plug it in, I've checked all power connections and my hdmi connection, but nothing will display on my monitor. I used tested the 7870 on a different 775 Motherboard and it works, I've put different gpus in the 1156 Motherboard (GT 240 and GTX 550ti) and they both display fine and boot to windows.
Any help about what's going on?
Thanks in advance

 

[Endless8]

When you tested your 7870 with different motherboard, did you use the same power supply?

 

[nicbread]

I said it above that I tested 7870 with a different Motherboard and same Antec 500W and it worked, but it's just with the 7870 and 1156 Motherboard when it doesn't work