This depends on how much you want to experiment. I have an 8-core mac pro, 3 mac mini's. a couple of Boxees, a Windows HTPC, and a 50 TB server. With respect to your mac pr
, your options are:
1) use the mac pro as your file server/ transcoding mule -- this is what I've ended up doing, after lots of experimentation. I use my mac pro to do the heavy lifting for transcoding for my iOS devices via Plex. I point the Plex library to my server and let it do the work.
2) give your mac pro the ability to dual boot into Windows, replace the video card with an HDMI-compatible one (you'll have to check to see if you can just plug it in without some work) and use it as a Windows HTPC (as well as a Plex server if you wish). I've done this as well. I use an ATI HD6870 for video, and can boot into Windows in order to bitstream lossless BR codecs.
3) just use your mac pro as a samba/afp server for your media playback devices. This is pretty simple.
After lots of experimentation, I chose the 1st route because it was just the simplest option, and allows me to still largely live in an OS X environment. Granted, I had some extra freedom with my 24-bay server, which I got and modified for a song.