Okay, I have 4 RAM slots in my computer - I'd stocked it with 2 2-GiB sticks, and 2 1-GiB sticks, but my OS only showed 3 GiB. This occurred on 32-bit Windows, 32-bit Linux, and 64-bit Linux.
The BIOS recognizes the RAM perfectly - I went into the BIOS settings, and everything displayed OK. 4 RAM slots, each of them said it had exactly what I'd put in it - total, 6 GiB.
Trouble is, the OS wouldn't see it, even a 64-bit one.
I'm not sure whether I need to enable PAE or something like that, but I'd really like my RAM back.
My computer is a Dell OptiPlex GX620, and I will provide any other specs/logfiles/etc requested.
Also, Memtest86+ returns a LOAD of errors when running it.