Quality components used in a 'conservative' configuration will last quite a while. I have OpenBSD Gateways that have been running for over 5-years with little more then a few fan replacements and dust removal. Still sporting that speedster P2-400 w/ 256-megs ram.
A quality power supply and good case cooling will be the most bang for your buck IMHO. Components will fail faster the dirtier the power and hotter the case gets.
Other then that, I usually stick to enterprise grade HD's and name brand quality hardware. My normal hardware mix is made up of: Asus Motherboards, Antec Cases, Antec SmartPower PS, Corsair memory, Seagate ES drives, nVidia/Intel Chipsets, etc. I like the motherboards with built-in video because it's one less component to worry about buying and failing.
Stuff I don't particularly care about quality on are the CD-Rom and floppy drive. If you build things right you will only need to use them once.
I also build my own boxes because the quality of most shelf-bought systems worries me.
As far as rotation goes, about once I year I call people up and sell an annual check-up where you show up with a compressed can of air and blow the computer out, check for bad fans, run SpyBot, etc. Also make recommendations for upgrades. I like to do it around christmas cause that's when most business' will have downtime for the holidays. Easy way to make a little extra change.
I'd recommend replacing all fans and HD's at least every 5 years. These are the only parts that move and will obtain the most wear. At a minimum they should be gone over in fine detail every 3 years.
