Tim_Axe,
Definitely not a population problem. I had all 4 slots filled at all times.
No other processors available for testing, unfortunately.
I also tried pulling out the new RAM modules and putting the CRIMMS back in, just to test that angle, too.

Nihil,
Will probably not get another chance to disassemble my box 'til at least Wednesday. Will try your recommendations then.
And yeah, RAM modules were definitely seated properly, as the little white clips won't click into place if the modules aren't correctly seated.
No, can't get into the BIOS, as machine isn't getting through the POST.

Not sure if it's of interest, but the MB has its own built on speaker! Which was just as well, because I couldn't find the correct pins (on the board) for ANY of the wires from my case. The manual for the board identifies the particular pin array as the "front panel connector", but doesn't specify which pins are for power switch, power LED, HDD activity, reset, or speaker. Thankfully, the box powered on by itself by toggling the PSU power switch.