All computers are not created equal. My Toshiba L305 could only access
2gig. I might put 5gig in that machine but alas only 2gig would show.
The machine may have 2gig but only able to access one. Has nothing to
do with the OS but everything to do with the address bus and hardware.

a simple search on the web using the motherboard identification will
give you this limit. The BIOS may be set-up wrong to access the memory
as well so needs to be checked. I recall the Ubuntu wiki had something
to say about memory setting in the BIOS just can't recall what.