Is the 64 meg module recognized when installed by itself? If yes, I must think your BIOS has determined the memory cannot coexit and has only configured the 256 meg when used together.
HP pavilion 7840 trigem-cognac mobo. ME. Calls for pc100 with 512 max. I have 256 pc133 computer recognizes and in my second slot I have 64mb pc100 which is not recognized. I've swapped the memory from one slot to the other with no change. Any suggestions?