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However, I do always purchase my cars new.
About the opposite of what I do - I buy new computers (most of the time) and used cars! The savings are significant - cars depreciate a whole lot more in absolute $$$$ in the first year or two than Macs do. 
My husband has that PB G4 and he's pondering a new one, but that's partly to do with the fact that he runs Framemaker, which must run in Classic in 10.3.9 OR under Windows, since Adobe dropped Mac support for it eons ago. He's starting to run into issues with web browsers etc. on the older OS. It wouldn't be so bad if he could run 10.5, but he can't do that and still use Frame - unless he gets an Intel machine and runs it under Windows. His copy of Frame is about 5 versions old now too.
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I don't worry about depreciation because I generally keep my vehicles for 10 years or more
I buy 'em used and then keep them until they're at LEAST ten years old! Our Corolla was 15 when we retired it last year, our Grand Caravan was 11, and our current cars are a very low-mileage 2001 Accord (was my mother's - it had 30,000 km on it when we got it last year!) and a 2004 Sienna. I buy low-mileage cars with superb reliability ratings and so far it's been good. The Grand Caravan wasn't in the "superb reliability" category, but it was a great deal and only a year old when we bought it from friends who were upgrading to the AWD version. We were lucky - apart from persistent door lock failures, until its last year it was very reliable.
Our Macs don't tend to last in this household quite as long, though some are working fine elsewhere with various friends/relatives! That said, we do have a fully functional 11 year old Lombard Powerbook (hubby uses it for iTunes mostly), an 800 Mhz Aluminum Powerbook (8 years old) which still gets used for movies and the odd bit of Solitaire, and a 10 year old G4 400Mhz awaiting a new home.