Except that only works if you already have a G4 Tower, and RedJack states he has a G4 Powerbook. The upgrades are also pretty pricey, so you're still looking at around $100-500 for the machine then another $100-400 for the ugprade, depending on which chip you get, and thats not including a larger HD, more RAM, better VGA...

Really its not as cost efficient as you might think unless you already own all of these things.