If you're looking at buying, not building, one...might as well buy a Dell that comes with Ubuntu pre-installed.
Dell & Ubuntu Website
They're cheap and have a decent warranty. Once you've played enough and figure out what
your needs are, then you can build it, and give the Dell to your wife/kid/Mother, or ?
Personally, I'd opt for the laptop with the 2.2 ghz processor in it. :)
Have fun!