Yes, I have a 1.1 2006 Mac Pro and it has airplay. Basically your Mac Pro has built in wireless and the OS X provides the airplay connectivity.
Coincidentally I have a Yamaha rxv 673 receiver and it works really well with airplay. Using my iphone as the remote it works a treat for music.
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Meant to add when your mac and Yamaha are connected to the same network the Yamaha will appear in the drop down airplay menu in iTunes.