Haven't tested it, but info is here CVar processAffinityMask - WoWWiki - Your guide to the World of Warcraft
If you use multiple wow instances then they will make use of multiple cores..., if you are talking of just one instance, why would you need multiple cores ?
Open task job manager. Rightclick on the process. Select Affinity. Choose what cores the process should run on. ... Dont get your hopes up for a performance boost. Windows might just be better at juggling cpu power than you.
Aren't those (5,6,7 and 8) HyperThreading (I'm presuming you're on an i7) and not physical cores? As others have said, Windows may do a better job of juggling cores than you.
There is no point to do this by hand. Windows will use the cores either way when he needs it. Sure it may seems that core 1 is taking all the load while the others a chilling around, but once you start stressing the CPU every core will have an even load.
I'm currently botting on 10 wow clients on the same time. And windows is using the rest. so this means windows is using full 8 if they need them