OK, I screwed up. Clearly I bought the wrong memory. I've got an older socket 939 board (MSI K8N) which accepts DDR2 (266/333/400). I had 2 Corsair sticks at 1GB each and all was going great for years. But I saw that prices were cheap, so I purchased some DDR2 PC2-6400.
I thought I could just slap the new sticks in and they would slow themselves down to whatever speed my mobo supports, but no such luck. This stuff won't even FIT in the slots. The notch is just a few millimeters off so I can't insert them.
I know I haven't been paying attention to PC builds in a while, but did they actually change the shape of DDR2? When did this happen?