On 28 July 2007, simon wrote:

...

I have recently installed centOS 5 on DELL pentium 2.7ghz (model optiplex GX270) and have 512 memory

I received the CentOS 5.0 DVD today. ASAP, I will upgrade from 4.4. After that, I will let you know how 5.0 sees the available RAM, in the same box.

The first place you need to look, is in the BIOS, to see how the box sees the RAM. If it doesn't see the RAM as available, CentOS won't either.