Windows 7 Ultimate 64.
Used to run Vista, and was perfectly happy with it, but 7 is legendary. And has DX11.
I'd love to run 7 on my study laptop as well, but sadly it'll have to stay with XP 32 as I'm not certain how well it would handle it.
Next project is a home-built NAS server, planned to run under Windows Home Server 2011
Not entirely sure on the exact specifics of the hardware, but something along the lines of an i3, 4 gigs of DDR3 and 3x1TB 7.200RPM drives which currently sit in my stationary.
Basically, it's a matter of moving my crap off my 800w stationary, and onto a low-power always-on solution.