When you finally decide it's time for a new computer, what does that mean? Do you replace a part at time? Do you perhaps replace a suite of parts (motherboard, RAM, CPU)? Are you the guy who replaces everything at once?
I would like to be that last guy (build entirely new system), but I'm more of the part(s) replacer/upgrader. I just never have the extra $1000.00 laying around to build a complete system.
Typically, I get the most mileage out of monitor*, chassis*, keyboard/mouse, speakers, printer and scanner. Everything else doesn't go longer than two years at the most before it's time for something new.