[FFL]

It does sound like you will benefit from portability. It also sounds like you might (eventually) want as much power as possible.

I'd consider a 12" iBook with maxed-out RAM (maxed out by you, not by Apple, of course). In a few months, you might be wanting more power, and by not sinking all your $$ into a PowerBook, you will be able to afford a G5 machine for your desktop.