I don't understand your current need for ESXi since you already have a server and an HTPC. If you wanted to have both but build one box it would make more sense. It seems like something fun to fiddle with, but not worth the cost it would take you to get it the way you want right now
If you are gaming from Steam you can use Ubuntu
Ubuntu Server LTS is pretty solid if you want a file server that you are capable of setting up from a command line. Plex, MySQL, NTFS drives, FlexRAID, etc are all available. NFS shares should kick things around the house nicely if you run a debian based distro for the HTPC. Steam is pretty much Ubuntu only, so you'll have to go that route for the HTPC (Arch support unofficial - https://wiki.archlinux.org/index.php/Steam
RomCollectionBrowser also integrates nicely with Steam, so your pretty set on the XBMC (all platform) and Steam (all platform) front. Not all games are available for Linux, but Steam is making a concentrated effort to put out the steam box which will be linux based so support is going to get better from already pretty good.
I don't really feel like you need ESXi for "occasional" Windows work stuff. If anything, that's when you could fire up the VM and run your already purchased copy of W8 specifically for that purpose.
The only point to keep you "stuck" with windows would be cablecard DVR. Myth (linux) supports cablecard content from the HDHR Prime, but only for non-HBO/Cinemax/Starz channels on Comcast and FiOS. If you could find a way to eliminate that from your "to-do" list then go for it. However, if a cablecard DVR is a true must-have, and you want to be able to watch/record HBO or
you're forced to use an archaic (inferior) cable provider like TWC, Charter, Cox, etc. then you should go ahead and suck it up.
Also, it's been well documented that Ubuntu is leaking your dash searches to Amazon, so don't go searching around in the Ubuntu dash (Post v12 versions only AFAIK
). In fact, if you avoid the unity interface altogether you'd probably be happier, since it's quite a pile of junk IMO