Depends what he needs to do with it, If it's just for word processing and surfing the web, then it's more than enough. My baseline Inspiron 1300 is fine for that and anything else I chose to throw at it. Though it would need a memory upgrade if I wanted to use it for heavy duty graphics or 3d imaging work.
As to what OS, I'd go for XP home as I really don't see the point of spending the extra for media centre unless you've got a silent PC with a large enough screen and quality stereo output to make it of any use.
Don't forget to buy some security for it, a cable lock should be a minimum.