If you have 2Gb worth of memory plugged in but the system is only
seeing 768Mb then there's definitely something screwy there !! How is
this 2Gb made up - 2 x 1Gb, 1 x 2Gb or what ?? What are the specs for
that Dell system regarding memory per slot ??

Have you tried running Memtest86+ and checking how much memory it can
see ?