If you are running vista and want to use Perfmon, you can go to the task manager performance tab and then hit the "Resource Monitor" button down there. It gives file access raw read/write speed and access response times. There are also more detailed cpu and memory charts in there. And of course, as fursphere noted, Heisenberg applies.

One interesting thing I notice on my machine at home is that after a cold reboot, before I do anything, vista spends several minutes pre-caching my wow data files into system ram. I guess it learned my habits and knows i will eventually start the game, this is also why I only use one single un-symlinked copy of the main wow folder - because it used to preload from all of them separately and I figured it was using up more cache with less benefit.