I think War is right, Count the little black ram moduals on each chip, if the top one (the one reading only 256) has twice as many as the one that reads correctly thats your problem.
PB G4 15" 1.2ghz - Pismo G3 400 mhz - iBook G3 500 "Jukebox"
"SawSmurf" G4 500 - Quicksilver G4 Dual 1ghz modding...