I always keep my OS on a single standalone partition for my clients computers for one simple reason. If your OS crashes and is not repairable ie. you have severe virus problems, you will be able to reformat that single partition and load a clean copy of your OS and not be forced to reinstall applications and documents, or in your case --> all 40gigs of WoW. Having WoW on a seperate partition than your OS will not affect performace due to it still being on the same HD.