Doesn't matter which OS is used to get system data, it's the same
hardware and that's what it reports. From your other post giving the
full readout you have 768Mb physical RAM in that Dimension 2400.

As for the amount of memory that a slot can take, this is determined
by the motherboard maker and on a Dimension 2400 it's 512Mb per slot.
Fitting anything larger will either lead to instability, failure to
boot, or the extra ignored - again depending on how the motherboard
firmware / BIOS handles the mismatch. Whatever the outcome, it's a
total waste of money trying it :-)

If you are 100% sure that you've plugged in 2 x 1Gb RAM sticks into
this Dell then I say again there's something definitely screwy but
going by the information posted you have 1 x 256Mb and 1 x 512Mb.

Memory testing is best run from a stand-alone bootable CD or flash
drive if the BIOS supports booting from USB devices. That way the OS
has no input at all in the matter