I know it won't go in a PowerBook G4 -- I had first dibs but, alas, it was no use. My uber-geek friend Rob (yes, I have friends geekier than me) kindly found the necessary info for me (because I was also having trouble with keyword searches) and indicated that for such memory compatibility questions a combination of Wikipedia and the Apple Support site (for confirmation, once you know the precise thing to search for, 'cause, well, Wikipedia) seemed to be the most efficient method of figuring things out.