It seems from what I read on the YAFU forum that the WUs max out at 64 cores.
So for those who have Quad Opteron boards with 16-core Opterons in them: eat your hart out, YAFU is taking full possession of your system!
Those with Quad board hosting 12-core Xeons -who are these people?- will see only 64 threads running YAFU, giving 32 threads the chance to do something else...