That option is just how much CPU the program requests. If there is more to use, it will use it. However, if you have another program running, that is the same priority, that requests some CPU power, Folding@home will only use 90%. I do this on my x1950Pro rigs. Since the GPU client still needs a little CPU power, I set the SMP client to only use 90%. When the GPU client isn't running it will actually use 100%, but once I fire up the GPU client the SMP client will go down to 90%. PS. I want my Quad-core back!