I just did this yesterday on my Chromebook. At first I tried the dual-boot with SD card, but that was a bit annoying, because (as you probably already know) most SD cards jut out of the Chromebook when inserted all the way, and for a dual-boot you have to, of course, reboot in order to switch OSes.
After fiddling around with that option, I ended up installing Xubuntu using Crouton, and it's amazing! This is the guide I used:
Didn't have much success upgrading Unity Precise to Unity Quantal or Raring, so if you want Raring (as I did), I'd recommend just install Xfce Raring straight away:
The big bonus of Crouton is that you can switch back and forth between ChromeOS and Ubuntu just with key strokes. I'm not sure on the mechanics of it, exactly, but it uses chroot to run both at the same time. It's not virtualization, and it's not a dual-boot, so you don't get any performance hit, and you don't have to reboot to switch.
sudo sh -e ~/Downloads/crouton -t xfce -r raring
I hope that helps.