How can I find out what RAM a computer system has? [duplicate]

Yes there is:

sudo lshw -short -C memory

It will output something like this:

H/W path      Class       Description
====================================================
/0/0          memory      106KiB BIOS
/0/4/5        memory      64KiB L1 cache
/0/4/6        memory      1MiB L2 cache
/0/d          memory      3GiB System Memory
/0/d/0        memory      1GiB SODIMM DDR2 Synchronous 667 MHz (1.5 ns)
/0/d/1        memory      2GiB SODIMM DDR2 Synchronous 667 MHz (1.5 ns)

You may also want to take a look at the full output of sudo lshw -short or even sudo lshw to get much more information about your hardware.

My thanks to jet and CYREX for pointing out a much better method:

sudo dmidecode -t memory

Which outputs something like this:

...

Handle 0x000F, DMI type 17, 27 bytes
Memory Device
    Array Handle: 0x000D
    Error Information Handle: No Error
    Total Width: 64 bits
    Data Width: 64 bits
    Size: 2048 MB
    Form Factor: SODIMM
    Set: 1
    Locator: M2
    Bank Locator: Bank 1
    Type: DDR2
    Type Detail: Synchronous
    Speed: 667 MHz (1.5 ns)
    Manufacturer: Mfg 1
    Serial Number: 1234-B1
    Asset Tag: Not Specified
    Part Number: SODIMM001

    ...

This method is based on the Desktop Management Interface:

From 1999, Microsoft required OEMs and BIOS vendors to support the DMI interface/data-set in order to have Microsoft certification.⁽¹⁾

It should therefore work pretty reliably.