The AMD Athlon 64 X2 5000+ Black Edition doesn't come with a heatsink/fan, which was news to me. "Well, I'll order one, and while I wait for it, I'll use the existing fan," I said.
Apparently, it runs just a teensy bit hotter than an Athlon 64 3800+. The 3800+ runs around 45°C. I hadn't even finished booting the computer before the motherboard started beeping at me: the CPU had already hit 80°C.
So it's back to the older, slower, single-core CPU until I get an appropriate HSF.