i have used: apple airport express, apple tv with disk, and apple tv without a disk. you can't go wrong with the apple airport express for music only. pick 1 up from crqaigslist for $50, just make sure it is 'N' capable which is the latest 802.11 standard. if you want video, get the apple tv. you can pick up an older apple tv with a 40GB disk for $50 or so. this will allow you to stream videos, pictures, and music from your imac. they also come with a little white apple remote. the newer apple tv allows you to stream netflix plus all of the above. i also hacked my older apple tv's and got boxee, hulu, and 50 other stations from 'g' to 'x' rated. the only problem was that the video quality was terrible at the time, maybe the newer hacks are better. i eventually reinstalled the standard apple tv software.
the added benefits are that you can control the apple products seamlessly using an iphone/ipad/itouch.
to hook this up, the airport express uses the mini-toslink adapter to toslink cable (you can buy a adapter for a stock toslink cable from fry's for $3). with the apple tv's, you don't need the adapter.