These 'books use DDR RAM which you can get 1GB sticks for - the older SDRAM models that'd be 512MB modules max. My wife uses a PB 867, which I picked up for $30 or something a good few years ago. Someone basically used a screwdriver to basically rip up the top lid to gain access to the HD. Thankfully, it bended back nicely. Then the onboard 128MB RAM module completely failed - so it doesn't chime - but boots fine off the 1GB stick in there, and never missed a beat, go figure.
JB