The task display in Win XP or Vista adds up to 100%, regardless of the number of cores. This X2 here shows 50% + 50% for two workunits running at 100% throttling (or none) and 100% CPU utilization.

Assuming a dual core, if there's nothing else in the system, yours set at 60% throttling should show something like 30% + 30% + 20% idle + 20@ idle.

That does not allow for system stuff, which could take a few %. BOINC throttling is a bit unsteady, so percentages will wander around. I also turn off the screensaver, saving a few more % for the workunits.

For a desktop, it's not that big of a deal. Throttling was invented to prevent heat buildup for a laptop. My cycles go to BOINC rather than idle.

BOINC also runs at low priority. If your other applications are well behaved, BOINC should back off if you want to do something else in addition.