menotu, I expected similar results to what you report, which is why I am somewhat puzzled.
I did a memtest and all is well.
I checked the BIOS and it has a setting to limit the OS memory available to 256MB or all of system memory to the OS.
I limited it to 256MB to see what effect it would have on this 2010 KDE install
yeah I know, but I could not reject the opportunity to see
I am posting this from KDE running in 256MBs
Mem: 248300k total, 233004k used, 15296k free, 4772k buffers
Swap: 7622792k total, 212488k used, 7410304k free, 55120k cached
well maybe 'running' is a bit optimistic, but it is functioning and for such as launching the browser it is not too bad at all.
The CPU in this is a P4 3.2 Ghz with Hyperhtreading and 64bit extensions and would appear to indicate it is capable of managing 4GB and more of memory.
There is no mention of reserving memory, in the BIOS, other than limiting the OS to 256MBs.
With that option off all the memory is supposed to be available to the OS ... according to what I read in the BIOS.
I will have another run-through the BIOS when I reboot ans reset the memory settings.