I installed 10GB but my 12.04 LTS 64-bit shows only 10GB. But the swap shows 10GB so I think internally it did find 10GB? I don't remember exactly how Ubunt calculate how much swap needs by default, per memory installed. I would understand if only 9GB is revealed to user, but 7.8?
total used free shared buffers cached Mem: 7975 927 7048 0 38 352 -/+ buffers/cache: 536 7438 Swap: 10228 0 10228