why not replace the 64mb by the 256mb , if they are not compatible together they probably will work apart .
Look at the bios settings and the sdram settings (latency )233 , 333 , 222 , perhaps there is the difference between the 2 ram modules . If the latency is to small , could result in errors .Try setting the three bios latencys to 3 . There is also a lot of cheap sdram on the market , some of less quality .Ask for brand on brand sdram , but anyway you never be completely certain of the compatibility with other brands
