Best to set maximum core usage to no more than 90% per client/core designation. This allows some 'headroom' for background processes.
For my present setup, I run a Pentium 4 Prescott 2.8 with HT, (ASRock Conroe 865) and I have two separate F@H processes active (different drives, folder names). No problem with operations, and can run other apps easily.
The F@H clients are designed to optimize their performance, yet readily cede to any user needs on the computer. The newer OS versions can control multiple-core (true) CPU types, and assign semi-dedicated processes to each as required. Best to allow the OS to relegate usage of processing per actual core, rather than manual settings. "Handing off" a process between cores can evoke delays and errors.
F@H.. to solve mankind's maladies.. in our lifetimes!