Airfoil is probably your best bet as far as software goes. It's an app that captures any sound from your machine and beams it to a variety of devices: iOS devices, other Macs, and Airplay devices such as an Apple TV or Airport Express.
As for the hardware, some hi-fi systems are starting to build in Airplay (which is the new wireless audio protocol Apple has introduce). If you don't want to buy a whole new stereo, I'd buy an Apple TV. $99 bucks for wireless audio and video is a pretty good bargain in my book. And, more and more apps and devices will take advantage of Airplay (including video Airplay) as iOS 5 rolls around.
You could also use an Airport Express, but that would limit you to audio only.