Depending on what mac model you have, you can use a normal 'windows' hardisk or RAM. Yo don't need to but the expensive Apple hardisk or RAM.

I'm using a PowerBook G4 867MHz and am using a 512mb third party RAM and a 80GB notebook hardisk which I installed myself. What model i don't remember but it works fine.

One tip though, just bring your IBook/PowerBook along if you want to upgrade the RAM. Install it there and then, boot it up and see if your OS could detect it or not. Play around it for like 15mins or so. If it does crash, chances are you got a wrong RAM installed and try another.