With the new CPUs coming out with 6 and 8 cores from Intel and AMD soon, you could now have one core dedicated to folding while gaming on the rest. The power draw of one core is getting lower and lower these days due to smaller architecture and the turbo core function causing only that one core to be supplied the extra power. Because you have so many cores it won't take away from the gaming performance. I'm planning on getting the upcoming 8-core Zambezi processor and I will fold on one core whenever my computer is on. I may also run BOINC on another core as well.
i say the more the merrier, if you want to use one core then i would use the classic client. more than one core i would use the smp client