Rogue Amoeba is a pretty cool company, but Do NOT get Rogue Amoeba's AirFoil if you want to watch TV shows and movies you purchased through iTunes. The audio from most TV shows and movies is unavoidably delayed from the video by 2 seconds or so - making watching the video impossible.
Sure, they have a good workaround for your home movies and such that removes the delay but this is NOT a solution to allow watching your prized purchases with external speakers over an Airport Express. Forget about using this to watch the media you really want to watch - like purchases from the iTunes store of great movies you love or TV shows you enjoy. You're stuck watching it on your Mac or connecting audio cables from your Mac to external speakers. Your AirPort Express system is rendered useless by this restriction that, I am sure, was forced on Apple by the TV and movie industry.
Airfoil is very cool, but it does nothing to get around this very annoying DRM BS.