The 261,xxxKB figure you're seeing is the real number of Kbytes in what
is generally called "256MB." The same goes for 523,808KB (512MB).
There's a little rounding error in the Bytes-Kilobytes conversion, too,
just to snooker those who try to figure it out...
The below assumes you have at least one and probably two 256MB RAMcards.
(Or at least you added 256MB worth of cards at the same time.)
Perhaps a 256MB RAMcard is not working. Look under the Performance tab
in Task Manager. If only half your RAM shows up there, then check what
BIOS sees.
If BIOS sees only half your RAM then either a RAMcard is not quite
seated correctly or is not working.
If BIOS sees all 512MB then W2k, which is extremely demanding of RAM,
has decided it can't use all the RAMcards at once. They are not quite
close enough to each other in performance. This does happen. You might
try moving each card to the slot occupied by the other, just in case a
very slightly different travel distance for the electrons in one
effectively balances the performance of both as far as W2k is concerned.
If you simply bought 256MB of RAM with no attention paid to matching
what was already there, that could be your problem. The speeds,
latencies, and so on don't quite match. Or you didn't check the manual
to make sure the slots in which the RAMcards are mounted are correct.