Firstly, this should be in the Networking forum (hence the red writing in this forum's description saying all Airport stuff should be in the Networking forum). If you read the sticky topics about Airport Express (of which there are about 4) you would have been able to answer your own question.
Secondly, you should be able to work it out from Apple's web site.
http://www.apple.com/airportexpress/airtunes.html
Basically to be able to stream music from iTunes to an Airport Express (AirTunes) you just need a mac with an Airport (or Airport Extreme) card installed and an Airport Express unit. No other hardware, like an Airport Extreme Base Station is required.