Get the small Airport Express from apple and you will be up and running in no time....send music wirelessly from your Mac pro to your existing gear.
It can't get any simpler then that. Plug it in (connect the audio to your amp with a 1/8 inch stereo to RCA adapter), use your airport assitant to set it up, launch iTunes and off you go.
As for formats and hard drive space. I prefer lossless compressed music rather than lossy formats although the differences are not always readily apparent - so don't let this stop you using iTunes and buying tracks from the apple store.
Hard drive space is getting to a competitive level with stacked CD changers. I use four Mega changers. A CD mega changer costs about $1 per CD of storage. Alternatively, 1 Gb of hard drive storage space costs roughy the same (you need roughly 1 Gb per CD of storage in order to have backup space as an umcompressed redbook CD takes roughly 500 to 600 Mb). I would suggest to use a RAID drive system or mirrored drive system with multiple hard drives to ensure backup. Five years ago when I built my system around Mega changers, the hard drive option was much more expensive...now I think hard drives are very competitive.
If you are a serious SOTA audiophile then you could try sending digital out to a DAC of your choice (rather than use the DAC's in the Express) or go for a SOTA slim devices transporter, which by all accounts uses some of the best DAC's in the biz.
G'luck
