I'm one of your BOINC to Fold converts, but have noticed that the WinGui client will only use 50% of an otherwise idle Pentium D. Poked around the settings, found a slider moved all the way to the right, but can't get it to use more than half the available resources. Othewise, glad to be aboard.
Some suggest that you should run two versions of the console program to keep all available thread execution pipelines busy; I'm not in favor of that if you're trying to process work units as quickly as possible.
I posted a more detailed discussion of Hyper-Threading from the F@H support forum here for those that missed it.