[alligator]

I'm going to be selling my G5, and I need to know what files I should transfer to another computer?

Mail for sure, but how do I transfer this?

iTunes, iPhoto, and document - those are a given.

What else can you recommend?

[TETENAL]

Your complete home folder and if you saved something outside that of course as well. You can use Migration Assistant and a Firewire cable.

[CharlesS]

Migration Assistant is, of course, great.

If you're doing it manually, I'd grab these things:

/Users
/Applications
/Library (if you've customized this one much - I usually have a bunch of fonts, screen savers, pref panes, etc. in here)

Aside from that, grab anything non-standard that you've put at the root of the drive and you should be good to go.