I have a Toshiba Satellite p25-s507 with two 256 sticks. While trying to upgrade for video editing I purchased a 512 replacement for one of them. The computer worked fine for a time then shut down completely. Finally figured to put the original 256 back in and it worked fine. Then purchased two 1Gb, installed them and, again, completely dead 'pute. Put the two 256's back in and it fires up fine. How come this happen, eh?