Alright, try this:
Disconnect everything from the mobo/PSU except the CPU, GPU, and one stick of RAM. If it doesn't boot, move the RAM stick to another slot. If it still doesn't boot, try another slot. Make sure to try at least one slot from each channel. (channels should be separated by color)
If that still doesn't work, move to another stick of RAM and try again.
If you do get it to boot on the first try, start adding hardware back in one piece at a time until it stops working, and you've found your faulty piece. If none of this works, it's likely the mobo or PSU. DOA products do slip through from time to time.
edit: also, double check to make sure all of those standoffs are in the right spots. If you put one in a hole that your motherboard doesn't use, and then put the board on top of it, it might be causing a short.