A quick update:

I ran memtest86 with each stick of RAM in the first slot individually. The first stick I tried got 3 errors. The second one got none! That one might be good, no?

The 4625 errors that I got before were because the test repeats, and I left it on for 7 hours.

I'm going to try with the sticks in the reversed slots of where they were before, make sure they're fully seated, and see what I get. But do you think that the one stick is okay then?? Is only 3 errors acceptable? The good news is that repeat testing brough up the same 3 errors, so as someone said earlier, it's not the motherboard, right?