Hard drive questions

I posted this at Buffyguide, where the computer savvy live, but it can't hurt to ask here (especially as it will only live for 24 hours there).

As I believe I've made perfectly clear over the years, I am something of an idiot when it comes to electronics. Robots frighten me. Computers are smarter than me. You know.

I am preparing to switch over from Windows to Mac. (I won't hear any arguments for either side; I have already made up my mind.) I wish to transfer my files from the Dell desktop (as well as two older PCs that are just sitting around) to an external harddrive, and then copy some of them onto the iBook I will be purchasing soon. Is this possible? I mean - can the external harddrive be formatted in such a way that it will be compatible with both operating systems? (Somehow I think maybe I'm not as stupid as I think, if I know enough to even ask.)

I could also use advice on brands, etc.