We just finished building our second all-in-one using the Thin Mini-ITX form factor and a box of off-the-shelf components. Although we still don't have the graphics performance we crave, LG's 22AM33NB chassis is a clean example of a productivity machine.
Does your child always want to use your PC? Is it time to build him his own? If you don't trust the tier-one vendors to sell you a well-balanced machine, you'll be happy to know that you can piece together your own entry-level box for less than $450.