I've googled and experimented a lot, but for some reason my Pi (model B, from one of the earliest generally available batches) is stuck with only 256 MB RAM. I've updated the firmware to the latest version, using rpi-update (http://hubcitylabs.org/unlocking-your-n ... of-memory/).
Whatever value I set gpu_mem in /boot/config.txt to is subtracted from 256. gpu_mem=16 leaves the OS 240MB:
$ grep -i memtotal /proc/meminfo
MemTotal: 237648 kB
I don't understand where the remaining 256 MB went. Can anybody help me?