Buy whatever you fucking want.
Turn the fucking thing off when you're not using it.
People are spending a lot of time building an energy efficient home computer, that can act as an HTPC, NAS, or whatever. It must consume as little power as possible, because it it will be on 24/7.
Why the fuck do you want to leave a system on 24/7 at home?
Unless you're unemployed you will be at work most of the time. And during that time, this machine is doing absolutely nothing.
There may only be one system that stays on 24/7 and that is your router, either some embedded router thingy or something based on a low-power PC, but that's about it. Turn every thing else off.
A machine that does 200 Watt idle that is only turned on when necessary will be more energy efficient than your specially build 40 watt NAS or whatever it is that is running 24/7. Nothing can beat a system that is turned off.
If you need a system, just use wake-on-lan or WOL to turn the damn thing on. It will be ready in about 2 or 3 minutes and then you can do whatever you want.
Fine if you leave a system on that is downloading some stuff during the night or day, but after the download has finished, turn the damn thing off.
If you're honest with yourself and really think about it, there is no need to keep a system on for 24/7. I know you may come up with excuses, but remember that you still can use your router for that.
By the way, if you're searching for a really energy efficient computer, buy a Mac Mini. Although quite expensive if you ask me, the're doing 30 watt in idle and almost nothing when sleeping. And if a mac is asleep, it can be woken with WOL and is up in seconds. They make an excelent download server.
There is also the option of S3 or S4 under Linux: suspend to ram or disk. However, your mileage may vary. If it works for you your system will be down and up in seconds. However, my experience is that it very much depends on your hardware if this will work. My Highpoint cards do not seem to like it, my array does not awake and the screen of the system stays blank.
If it works, it is a faster solution than just to turn the system off and on with WOL. My experience is that is seems not that robust.