I have a current generation base model. i5 1.4ghz with 4gb of ram.
People tell you to have the highest spec machine you can, but TBH I think that's as much bollocks as audiophile USB cables! (and I work in IT for a living).
My little mac mini, running Roon Server, pushing different audio to four zones, while I browse through albums on it, consumes abut 3-4% of my CPU power and about 260mb of RAM.
Its very resource light, and will run on pretty much anything really. You only need bigger power and graphics if you want to run the GUI. The actual backbone of roon is amazingy light an well put together.
It has simplified so much about what I do. I had previous tried the following:
ITunes on its own - no bitrate switching, and I had to keep changing audio source when I wanted to watch Netflix / movies through the HDMI and my TV.
Amara - not free, so expected better, but gapless audio didn't work, bitrate switching needed huge buffering and the whole experience was buggy and laggy, with no decent remote function.
Audirvana - decent sound and bitrate switching, but in exclusive mode it totally took over my system, meaning that to watch movies through HDMI I needed to actually come completely out of audirvana and iTunes to get my audio core back.
Roon - takes ownership of my DAC in exclusive mode, gives me instant bitrate switching with no lag or delay. My default mac audio source of LG-TV sticks and I can just open up a browser and start watching movies, with absolutely no penalty for having Roon running as well.
In fact you could actually be in another room playing music on Roon, while watching a movie on the mac mini through a TV, and this all works utterly seamlessly.
Add to it, a sublime interface through the remote app and I think Roon is a no brainer at the moment. It is by far the best solution for my needs that I've found yet, and the least buggy. It allows me to get the best audio performance out of my mac mini with no real effort, and still lets me carry on using the mac as the centre of an entertainment system.
Its not cheap, but its worth it!