As I write this, my computer is doing many, many things.
It's running EVE Online - I'm mining, it's minimized, it's just burning CPU while I burn away at an asteroid. Every half an hour I check it and change to a new asteroid if necessary. In terms of personal effort vs money, it's hard to beat this return ratio.
I'm doing development work on it. So every once in a while it spikes when I compile.
I'm playing music constantly, of course, so Winamp's running.
I'm decompressing a large set of files.
And I'm running two entire virtual computers, both of which are doing nontrivial things.
Yes, I overbuild computers. But I'm capping both CPUs right now, and using most of my RAM. And it's holding up, and still has no speed issues, everything's performing like it's not under any load because of how I have it set up.
(Well . . . except Eve Online, but I have that turned down.)
And that's why I overbuild computers - because I can *use* this much power.