New computer won’t boot

I've recently purchased a set of components and set about building my own desktop. It is not booting and I am stuck.

When I hit the power button, the fans spin, but that's all. There is no video. There are no beep codes.

• Power Supply: Rosewill RD700 700W ATX12V V2.3
• Motherboard: Biostar TA970 AM3+ AMD 970 ATX AMD Motherboard with UEFI BIOS.
• Processor: AMD Phenom II X4 965 Socket AM3
• RAM: 2 x 8GB 240-Pin SDRAM DDR3 1600
• Video Card: Sapphire Radeon HD 6670 PCI Express 2.0 x16

I have tried swapping out the power supply. My backup power supply is 850W. Both power supplies are brand new.

I have tried swapping out the processor. My backup processor is an AMD FX-4100 AM3+. Both processors are brand new.

I have tried swapping out the video card. My backup video card is a Sapphire Radeon HD 4650 PCI Express. It was working in the last computer it was in. This motherboard does not have on-board video.

Because I was able to swap out all components other than the motherboard itself, I thought that the motherboard must be faulty. I RMA'd it and got a new one, of the same model. This did not fix the problem.

The monitor is known to be good; I am using it to ask this question.

I have verified that both the 24-pin motherboard power and 4-pin CPU power cables are in place.

I have 2 pieces of RAM, each 8 GB. I have four RAM slots. I have tried the RAM singly and in pairs, in all fourteen possible combinations of RAM + Slot. I have tried this RAM in another computer, and it was able to boot Windows.

I have other components, but I have not yet begun to connect them. I assume I should be getting some video or beeps or something, even if I don't have a hard drive hooked up.

I don't know what else to try.