The windows machine will probably stay, and I'm pretty happy with it actually. It's a low-power Intel ATOM 1.6 GHz dual-core machine, which has similar power usage as a Mini (about 45W). The upside is that it can handle two 3.5" hard drives, so storage capacity is bigger than what any Mini can do. That windows machine only serves two purposes:
-Duplicate iTunes library (about 1.6TB at this time), which is always on for use by the AppleTV devices or remote play to iOS devices. It is kept in sync with my main library (currently on the Mac Pro) using Home Sharing.
-Live backup device using crashplan backup I have setup with a friend. Essentially everything I put in a certain directory is backed up to his crashplan server and vice versa, so we both have off-site backup.
Both of those functions could be performed with any Intel Mac Mini (and at one point in the past, were handled by a Mac Mini), but I prefer the option of having 3+ TB of fast, internal storage.
I'm pretty certain I will sell the Pro in the near future. I'm just not sure what I'd replace it with, if anything. My RMBP is faster than any Mini currently available, so it would seem to be a bit of a regression to go that way. I'm leaning towards USB 3.0 as my external storage standard rather than TB, so I don't really want to buy a current mini with USB 2.0. If a new quad-core mini with USB3 and good discrete graphics is announced shortly (i.e, at the same time as Mountain Lion), I would be very interested to try that out. Or if a retina iMac appears out of nowhere, I'd jump on that, but I highly doubt they can pull that off technically.
I'm well aware of the power and heat issues with large desktop computers. In parallel to my Apple setup, I have a Windows machine with a 6-core Sandy Bridge-E CPU (overclocked to 4.4 GHz), 32GB RAM and dual ATI 7970 video cards. It is used for gaming and video encoding/transcoding (and blows any Apple machine out of the water, which is a bit of a shame considering I built it for about the same price as the current base Mac Pro). At peak CPU and GPU load, it pulls about 1kW from the outlet, and requires a portable A/C unit to keep the room cool in summer (on top of the central it for the house).