You might have the problem that the Graphics card that you chose might be UEFI Bios only. I recently bought a used XFX RX460 for my GA-P45T-ES3G 775 board and it works but when it boots I get this weird flashing screen for a few seconds then it goes on, (I think it is looking for the UEFI BIOS) Make sure you have the latest Bios in your 755. Read where there were some Saphire cards that had switches or buttons on them. I put a oem Dell RadeonHD6350 in my SC430 server and the only drivers that would work were from Dell. You may have one of those wonderful prebuilts that hates AMD GPUs for some reason. I think I lucked out by getting the XFX version.