A few things to try: just use a single processor and DIMM, if you currently have more than one installed. if that doesn't work, pull all memory, see if you can get the board to power up and give you three beeps. If it does, you have sufficient power for the processor(s) at least. The power supply should be capable of outputting 24 amps on the 12V rail, peaking up to 28A on boot, potentially.
Thanks for the reply!
/* just use a single processor and DIMM */
But I already am.
The power supply is ok, it's new, it's 400 W, and my CPU's max TDP is 135 W.