Look at the Airfoil app for the mac along with the Airfoil Touch or Airfoil Speakers app for the iPod Touch/iPhone/iPad in the app store. However, while you are streaming to the iPad you won't be able to run anything else at this point in time.
There is also Simplify however they are discontinuing support so I don't think it is available anymore and their server will be going offline at some point. Added to which you can stream older iTunes files that are DRM'd.