Well, I feel very fortunate. I have a desktop that I built with my father-in-law three years ago. I just upgraded the memory to it's max over the summer so I'll be putting anotherone together soon.
My notebook is three years old and running great.
I replace items as I need to. I have a couple monster hard drives and keep most programs on one and files on the other. I will have to replace the keyboard soon, as I tend to bang-the-keys and wear them out.
I attend some after-hour BS sessions with some folk who keep up on all the stuff so I can just ask what the newest and best things are. I tend to wait until programs (I.E. Windows, MS Office, etc) have been out about six months before upgrading.
I really want a Mac, but need a couple more clients first!