Back up anything you care about, then just dive in.

If your a linux user, assuming your hardware cooperates, then this shouldn't be a big deal for you. You know the drill, something doesn't work right, you google, you try this and that, rinse, lather, repeat.

BTW I would suggest you use EasyBCD instead of Ubuntu's grub, just use the advanced option at the end of the Ubuntu installer and tell it to install to the partition instead of MBR then set it up in Vista.

Also, while using one of the available distros is good initially, just to figure out what you need to get your hardware working, once you know which "third party" drivers/mods you need, you should definitely consider installing from the retail DVD, making only those changes necessary to support your system.

And last but definitely not least Little Snitch is a MUST.

Good Luck.