system monitor is reporting 3.9 GiB only? [duplicate]

Possible Duplicate:
Why is more than 3GB of RAM not recognised when using amd64?
Why does Ubuntu only show 3GB of RAM?

my ubuntu 10.10 x64 was installed on this box one year ago, with 6Gb of ram.

recently i've noticed that system monitor reports just 3.9GiB for memory. That can't possibly be right (in windows i'm having access to the full 6Gb of ram on the same machine)

any idea what might be happening?

EDIT

uname -m
x86_64

 free -m
             total       used       free     shared    buffers     cached
Mem:          3954       1802       2151          0         53       1014
-/+ buffers/cache:        735       3219
Swap:         5721          0       5721