I've got 4GB on my machine (two cards of 2GB)
although the command
free -m says:
total: 2001 used: 1816 free: 185
I've got 12.04 and I think PAE should be installed and enabled automatically.
Why is that?
Two things may be happening:
1 - Either you have installed the 32 bit version of Ubuntu (i386) which is unable to "see" 4 gigs of RAM or more (Actually, 32 bit oses usually see about 3,5 gigs of RAM, including Ubuntu, unless you have installed and enabled the PAE - Physical Address Extension enabled kernel, in which case it can see all 4gb, all the way up to 64gb, at least in theory. For information on how to enable the PAE kernel please see:Enabling PAE). If you would like native support of your 4gb, and/or planning to add more in the future, you are better off installing the 64 bit edition of the Ubuntu 12.04 (should be under a name x86_64, or x64, or amd64). If you decide to do so, do not forget to backup the files that are important to you before doing so.
2 - Make sure that the dimms are installed properly. What may also have happened is that one of the two dimms may probably be problematic, in which case, to determine which one of the two is problematic, remove each one of the two and see if the system responds properly to the change. If none of them appears to be problematic, go to solution #1.
It looks like your machine is only detecting 2GB of ram.
First open a terminal, then run the following command and edit your answer to include the output.
Has the additional ram been installed recently, and can you confirm it has worked previously?
You may simply need to remove and re-install the ram making sure it sits properly in the slot..
pull the power cord first the push the power button to drain the system caps of charge wait 5min then pull them and use a ground stap for wrist or keep idle hand touching metal, to prevent ESD damage.