XPS 8300 will not recognize new, verified working HD 7750 cards

I've tested it in another PC in the house, but it will not be recognized by the XPS 8300. This originally came with a decent card, I believe it was a 6770. However, that card has been acting up, overheating and giving problems after 2 years of intense use so I decided to replace it with a 7750 for similar performance (at least on passmark).

I've uninstalled the drivers I already had, and hooked it up. It's simple, no additional power needed, just shove it in the slot, and I get no display at all, not even during BIOS. I've double checked that all the drivers from the old card are gone and even tried a different radeon hd 7750 from my other computer, the XPS simply will not recognize either of the working cards. I've unseated and reseated it multiple times and dusted out the slot. 

Is there anything else I can try? Both cards were verified (today) to work. I'm not even trying to crossfire them, just trying to get one of them working. 

Edit: Not sure if this matters at all, but one is manufactured by MSI and one by HIS. My XPS 8300 has a 460W PSU which should be more than sufficient.