When I told my parents the news about my house, my mom immediately claimed the guest room as her own, half jokingly. She's wanted to relocate back to FL from AZ for some time now, but isn't able to. Other than a 2.5 year period, since college I've always lived solo. Last year I let my sister move in for about 10 months since she was pregnant and struggling. Mom came to visit for a few months to help out, so my house was packed. Coulda used some of those extra rooms you have
The game room sounds cool, if I had a 4th bedroom, I'd probably do something similar. I mostly play racing games on my PS4, and have a pretty simple racing rig since it has to be easily stored in the living room and not take up a lot of space. I wouldn't mind having a more serious setup, and put 3 curved screen TVs on a mount, to give a surround view sort of feel.
From time to time, I still keep tabs on those R710/R720 offerings on Ebay. I both thank and curse you for that. I usually don't look at the auctions, but I happened to see an R710: 2x 2.5 ghz 6 Cores, 32GB RAM. Ended up going for under $100 out the door, with only 1 bid on it.

I think I have all of the cables for my HTPC ran through the attic that I'll need, barring a drastic change in my current setup. Had a friend over, and he suggested I move the cable box, PS4 and surround sound receiver into the server closet after I get the rack put in, run an IR repeater, and get those components out of the living room. Hate it when friends feed your mod bug.