Thankfully, I believe I have located the problem. The motherboard I bought is an Asus P5B Deluxe, and several commenters on Newegg noted how they used Crucial 1GB memory sticks inside of this machine. I thought I was all set.
But a little-known voltage setting is causing an issue. I have never heard of needing to mess with this, but apparently the RAM I’m buying is on the bleeding edge or something. The motherboard is set to use RAM that runs on 1.8 volts, and the Crucial RAM uses 2.2 volts.
The motherboard works with 2.2 volt RAM – just go into the BIOS and set it. But you can’t get the BIOS working until you boot once with a 1.8 volt RAM stick. Chicken-and-egg situation; the only solution I could come up with was to buy a cheap memory stick for a single boot (cost = $32) to get everything working.
Hey, it’s cheaper than trying to return a bunch of stuff to the manufacturer and buying new stuff. Or dragging the PC to a local shop for help ($70). No friends have a spare DDR2 RAM stick to get me going.
I also got some other recommendations (via the motherboard manufacturer), along with the simple advice of “Hey, take out the memory and see if that solves the problem” so I have plenty of things to try to get it working.
Now I just have to hope that something will actually, truly, work.