You can also use lshw to check what is installed-
*edit make sure you are running lshw as su/sudo, it returns less info if run as a normal user.
Code:
*-memory
description: System Memory
physical id: 25
slot: System board or motherboard
size: 4GiB
*-bank:0
description: DIMM 1800 MHz (0.6 ns)
physical id: 0
slot: A0
size: 2GiB
width: 64 bits
clock: 1800MHz (0.6ns)
*-bank:1
description: DIMM 1800 MHz (0.6 ns)
physical id: 1
slot: A1
size: 2GiB
width: 64 bits
clock: 1800MHz (0.6ns)
*-bank:2
description: DIMM 1800 MHz (0.6 ns) [empty]
physical id: 2
slot: A2
width: 64 bits
clock: 1800MHz (0.6ns)
*-bank:3
description: DIMM 1800 MHz (0.6 ns) [empty]
physical id: 3
slot: A3
width: 64 bits
clock: 1800MHz (0.6ns)
Here I have 2 x 2GB sticks.
lshw is also handy because it will show you the 'real' amount of installed memory, not 'the amount of installed memory minus any memory shared for onbaord video, etc. use'.
Code:
MemTotal: 4059612 kB
MemFree: 1395624 kB
Buffers: 50852 kB
Cached: 997596 kB
SwapCached: 0 kB
Active: 1501404 kB
Inactive: 919096 kB
Active(anon): 1378956 kB
Inactive(anon): 5328 kB
Active(file): 122448 kB
Inactive(file): 913768 kB
Unevictable: 0 kB
Mlocked: 0 kB
SwapTotal: 4194300 kB
SwapFree: 4194300 kB
Dirty: 5664 kB
Writeback: 0 kB
AnonPages: 1371984 kB
Mapped: 124956 kB
Shmem: 12208 kB
Slab: 69396 kB
SReclaimable: 40232 kB
SUnreclaim: 29164 kB
KernelStack: 3824 kB
PageTables: 84452 kB
NFS_Unstable: 0 kB
Bounce: 0 kB
WritebackTmp: 0 kB
CommitLimit: 6224104 kB
Committed_AS: 3872200 kB
VmallocTotal: 34359738367 kB
VmallocUsed: 316032 kB
VmallocChunk: 34359402492 kB
HardwareCorrupted: 0 kB
AnonHugePages: 235520 kB
HugePages_Total: 0
HugePages_Free: 0
HugePages_Rsvd: 0
HugePages_Surp: 0
Hugepagesize: 2048 kB
DirectMap4k: 60932 kB
DirectMap2M: 3082240 kB
DirectMap1G: 1048576 kB
4059612 kB = 3.87GB (OK, 3.87GiB for those that like XiX sizing), not the 4GB that is really installed. That 3.87GB figure is accurate (as far as I know) for what the system can actually use though.