Half of RAM does not load on boot

Running NT Server; P3-450; 3 RAM slots

Recently my 128M of RAM (single slot) just
started showing 64K at boot up. I thought the
RAM was bad, so I replaced it (and upgraded)
with three 256M RAM. It should have shown
768M of RAM, but only shows384M, exactly
half again. Obviously not a seating issue.
Anyone have any clues?

thx in advance
tim