These are the 3 things I find often run my current system (2.4 GHz Celeron, 2 Gb RAM) to 100% CPU.
We also have 6 users logged on all day, and the younger ones sometimes leave Web games running in their browser window.
Win7 experience index rates my system as 2.0 out of 7.9 (low value was 2.0 for graphics, high value 5.9 for disk).
So, we don't run anything really heavy duty for long periods, but any of the above seem to bump the CPU to 100% and the system becomes non-responsive.
I read a lot of good comments about the E8400, but this is listed in the FAQ as one to avoid now. I just want to get a good ASUS board and multiple core processor that will hum pretty good in a home system for the next couple of years.
Oh, I also want it really QUIET, and don't want to hear much more than a spinning disk and CPU fan (I have a Zalman running at about half throttle currently).