IMO, if you get a desktop + laptop, it's because the desktop has features or power just not feasible with a laptop, or because one machine will be unusable for periods of time (compiling, rendering, family member borrowed the laptop for a couple days, et cetera).
For that machine, most of the benefit you'd be getting could be gotten through peripherals. Get a monitor to hook it up to (you'd have spanning, better than a "standard" desktop setup...), a firewire enclosure or two (superdrive, hard drive), speakers, keyboard/mouse, et cetera. Plug it all in and you've got yourself an instant desktop when you come home.
Only thing I see an issue with is iDVD not recognizing external burners. Keep in mind, though, dvd burners can be purchased for a internal upgrade, although I don't think they're that cheap.
Overall though, I think creating a great desktop environment for your powerbook is a better deal. Comes out cheaper in the end, unless "looking for a tower to compliment it" means "I need a computer for the kids".