I am building a new system, DH87RL (BIOS v322), i3-4130, Radeon HD 7790, and I have the exact same problem, and it is reproducible easily.
All you have to do is pop in a PCIe graphics card, and set "Legacy Boot" to OFF. The system will not boot, and will not produce a video signal, until the PCIe card is removed. I have tested this with an ASUS Radeon HD 7790 and an MSI GeForce 7300LE. The only way to resolve this is to leave Legacy Boot ON.
This is, it almost goes without saying, entirely unacceptable, and needs to be fixed immediately.
It also appears that this issue causes Window 7 Install to freeze at "Starting Windows". I have verified that with Legacy Boot enabled, you can run the Win 7 EFI installer from the BIOS internal EFI shell, to be sure you are running the EFI installer, and it doesn't freeze. If you take out the PCIe card, and disable Legacy Boot, it freezes every time.