I get most of my memory now from newegg. It's just more convenient because I'm usually buying everything else from there anyway. Kingston Valueram is usually my choice since it's a lifetime warranty from a company that probably isn't going anywhere and the chips usually tend to be from known (to me) manufacturers. It also doesn't have flashy heat spreaders which add cost and probably little else (for me).

One thing to be cautious of in laptops is probably the dimensions of the memory module. I consider a normal laptop module having a height that's not much higher than the notches cutout for the module socket in the laptop. I have seen many modules that are about 1/4" higher than that with additional chips mounted horizontally in the extra real estate. This could be the reason (especially on a 1GB module) for the different part numbers. The Dell may have room for an "extra height" module while the Powerbook might not. Hence, the different part numbers. I've never seen what the Powerbook's memory bay looks like, so I don't know if that's the case. Seems possible though.