Definitely look into a cooling upgrade. I use the Xigmatek Dark Knight (S1283V) with my i7 and it works well overclocked or not. Actually, if you are still using stock cooling you might be experiencing the i7 underclocking itself due to overheating EVEN at stock speeds. Five copies of WoW running on a single processor can stress it even more than running a game like Crysis.

As for performance, it should generally be pretty well in pre-BC areas. Check to see you do not have MSAA set too high. It is around the resolution switching option, not in the video option page. Affinity should be ok with all set to use any core, but definitely monitor task manager while playing to double check it is working properly for a while.

If you are seriously thinking about putting more cash into a video card or such, consider building a second system. Even moving a single client (the tank) off to a cheap dual core system would seriously improve bandwidth to the other four instances. Personally, I use four dual core machines along with my original i7 box I built back in December. This setup will be patch-proof and takes any performance issues out of the equation.