Thanks for the help! I'd actually read that thread, but didn't (quite) follow all the steps -- I'd not disabled all the legacy USB support. A little more meticulous step-following, and I believe I've found what is preventing my computer from booting....

I had enabled the legacy USB support for keyboard. Without it, I can't CTRL-I for the Intel Raid configuration, nor can I use my keyboard in Memtest86+. I can use the USB keyboard fine within the BIOS, of course. However, with the legacy USB keyboard support enabled, my system will not reliably boot at over 400MHz. With the USB keyboard support disabled, my system behaves as I'd expect. So, I'll disable the legacy USB keyboard support in the bios (I have a PS2 keyboard that I can use in a pinch).

This gets me past my current frustration point. Before I ask for any more specific help, I'll bring along the screenshots from CPU-Z and Easy Tune. And in the meantime, I'll follow the suggestions already posted.

Thanks again!