How can I play music WIRELESSLY from my Mac Pro?

Sorry I can't answer how to hook up with wireless speakers (perhaps someone else will pipe in)

BUT: If you have a regular audio system in your house (receiver, wired speakers) you can't beat Airport Express (check Amazon or Apple Store). (Why fork up for wireless speakers - the sound quality on these tend to be inferior anyway)

You just attach a 3.5m->RCA (or optical cable) between the Airport Express and your receiver. Wait till you hear your iTunes off some real speakers.

Call Apple or read AE manual before diving in, to make sure it'll meet your needs..

HTH

Apple TV or Airport Express. Plug that into any amp in any room in the house or tv for that matter.

whoops, forgot about Apple TV. That's an excellent option. The question would be which version.
The prime advantage of ATV is that you have an onscreen interface AT your party or in your living room, rather than choosing a playlist on your computer.

I have the "Old" Apple TV which I LOVE. ( _ http://www.amazon.com/Apple-TV-160GB-Hard-Drive/dp/B000RQHAUA/ref=sr1_2?s=electronics&ie=UTF8&qid=1289178072&sr=1-2 ) $149 at Amazon.
It's big compared to the new one but has a full complement of outputs (HDMI, component, optical, RCA). More importantly, it has a hard drive, so you can transfer your music & vids to the ATV's HD. You can ALSO stream directly from your computer, but I believe it's slower than the new model. Personally I prefer having the hard drive, so I don't need to be using the computer just to stream. BUT bear in mind there will be no further software updates for this model

The latest Apple TV one is cheaper ($99), streams music and video. It's teeny, doesn't run hot, But only has HDMI. My only concern is that it is SO new. You may want to check these forums to make sure that all is working well. It has some additional capabilities (I think including Netflix - but I'm not sure, so check it out if you're interested.

Good luck, seems you have several options to confuse now...