I do that currently with a MacBook Pro and a Dell running XP. The link between them is a website / synchronizer called dropbox (www.getdropbox.com).
Dropbox let's you set up a free account on their website as well as local clients on all your machines. These clients then keep a folder sync'd (in Windows, it's called My Dropbox in the My Documents structure; in OS X it's Dropbox in your home directory) across all your computers.
Then you just put your brain into a folder inside the Dropbox folder and open it in PersonalBrain. Changes you make will be sync'd automatically to your other machines.
For safety's sake, though, you should probably try to only have one PersonalBrain running at once (I think the license requires this, too) to avoid duelling updates.