Memory recap...
After installing that 512MB DIMM again, everything seems to be running fine. No weird application crashes or anything. I suppose my GamingPC is going to be just fine, now.
The only thing I really did differently from last time is that I moved it to the first slot instead of the last. Seems to have fixed it. We'll have to see, though. Oh, we'll see.
You know, I seem to remember something funny about chipsets that support the Athlon XP and DDR 400 and how there was something weird about where you needed to put a big DIMM like that. Maybe that's what it was!