Time Machine question

[lamina]

I have a 17" iMac downstairs, and my PowerBook wherever I am.

I will definitely be using Leopard on both (bought the family pack). Is it possible to do a backup onto the iMac from my PowerBook if I leave a USB drive plugged in?

 

[Telp]

I am a bit confused, i know you can do it wirelessly, though im guessing slightly slower.