Most people use a computer for an information gathering device or mild entertainment (kind of like a TV). This would include simple games, mp3's, video, email, web browsing, instant messaging, etc. Any system made in the last 5 years will run those aplications with ease, some could even do it on much older hardware.
If your using the plain vanilla "commodity application" you might as well get a commodity PC and OS to go with it since a high end PC, Amiga (whatever that is except lots of money and little horsepower), or Mac is just a waste of your money. Do you need a Ferrari if all you do is drive 3 blocks to the local McDonalds and back at 35 MPH?