Update: The mystery has been solved. Thanks to everyone who wrote in.
Despite the 4× 1Gb RAM modules I added to the Asus A8V Deluxe mainboard of one of my machines, the BIOS (and Linux) only sees 3072 Mb. The BIOS is up-to-date (version 1018.002).
The specs say that memory is
4× 184-pin DIMM, Max. 4 GB, DDR 400/333/266 SDRAM, ECC, Non-ECC, Un-buffered
Dual Channel memory architecture
The four slots are labelled A1, A2,
B1, and B2. A1 and
B1 are blue, the other two are black. All four chips
are DDR400 CL3, even though they are by different
manufacturers.
With all four slots filled, I get 3072 Mb of RAM. If I remove either of a black module, the RAM falls to 1024 Mb. If I remove either of a blue module, the RAM falls to 2048 Mb.
Any idea what is going on? How can I get the full 4Gb I want? Please write in if you know.

