I fix pc's for a hobby so have 11 Boinc machines running right now, and 2 non Boinc machines. Of the 11 Boinc machines 2 are 6 core machines, 4 are 4 core machines and the rest are dual core machines. The 2 non Boinc machines are a 4 core Mac laptop my wife won't let me use and her dual core work laptop I won't touch. All but 3 of the 11 have gpu's that ALSO crunch in them, 2 of which have dual gpu's in them.

I got MOST of my pc's by replacing firends pc's and they gave me their old parts which I now crunch with. I also pick up pc's on the side of the road and after wiping the hard drives use them too. I have over half a dozen pc's needing very little work to get them up and running sitting on the floor but my house is maxed out on its electricity usage, some of the outlets were getting EXTREMELY WARM so I had to unplug 2 pc's! Working on peoples pc's sometimes they give me cash and I do use that to buy newer parts, such as the 2 6 core machine I built for $400US each. At Christmas I only ask for cash, or specific pc parts, and get enough to buy parts with that too.

I have not been able to get others to crunch too though, they just don't see any short term possibilities of finding anything worthwhile. I have been crunching since 1999 and am #72 in the World so won't stop, but just can't seem to convince others to join me in the search. My old work considered it, with over 10,000 pc's they would have been a FORMIDABLE Team themselves, but the local power company offered to cut their rates if they ran a piece of software that shuts all idle pc's off at night. This saved the power company from having to build a new power plant and saved my old work a TON of money too!

1: 8 machines, 32 cores. All Q6600's.
2: Purchased 4, built the other 4. (Going to purchase stock in my Electric Co.)
2a: 3 Linux, 2 Win XP, 2 Win Vista, 1 Win 7.
3: I'll stick with these for now.

This is an interesting question, but perhaps a bit simplistic. I have a total of 42 threads. 8 are on an i7-2720QM machine which I had to stop using, as BOINC blue-screened the machine due to temps even with TThrottle. So, now 34 usable threads. (BOINC as the culprit was arrived at after full hardware testing and the removal of elective software.)

But, starting with my oldest machine, an i7-920, one can not simple run even desktops at 100%/100% of CPU's. Heat is the problem.

TThrottle @ 55 deg C helped with an i5-520M.
My i7-3930k has sealed liquid coolers.
My i7-840QM is on TThrottle @55 deg C.

No machine are at 100%/100%.

Not necessarily true. 6 of my machines are 24/7 and all of them have stock coolers. Two of them are servers which are made for 24/7 running at full speed,, the others are desktops. I make sure I blow them out periodically and do not let the dust bunnies build up in them, especially on the heat sinks and all of the external vents. With good airflow, although the exhaust air is warm, they stay cool enough to stay functioning. Of course, if you overclock you will need better cooling.
Cheers