I tend to buy and keep stuff till it dies on most things. I do it with clothes, shoes, cars, etc. Sometimes I can still hear my mother telling me something like "you don't need new ___(shoes, bikes, etc), your old one still works fine" when considering buying new stuff. My main TV is still a non-HD tube model because it still works.
I tend to be this way with computers, as well. My late 2005 era Powerbook still works fine. I still have a P4 era XP machine that works fine, and is about to be converted to some flavor of Linux. I have splurged a bit, though. I bought a new gaming PC last year, and I just bought a refurb Mini. I did manage to pay for half the price of the Mini by selling my old Core Duo Mini to a friend.