We have a large collection of pieces floating around (cases, power supplies, video boards, RAM, etc.). Usually, when it's time to build up a new machine, we buy a new motherboard and processor, and we usually have at least some of the other stuff (video, ram, hard drive, case etc) or an old machine we can retire.

I generally go top shelf on motherboards, power supplies, and RAM because I learned long ago that the janky $50 Fry's special motherboard, or the bargain Yun Ho Fluk brand power supply for $15 (when a good one costs $75) nearly always results in an unreliable machine.

If I build up a machine, I know exactly what's in it and if something breaks, where the problem likely is. If I buy a Dell, I have no idea what motherboard, RAM, or power supply is in there, and it probably, in any case, isn't the same top shelf stuff as if I go and research and pick it out myself.