I've recently put together a very low wattage desktop PC for my day-to-day needs; it may be of interest to you.
Over the years I've aimed for a low power consumption desktop which would perform well as a general-use computer. I gradually moved away from a PC to laptops, most often with their screen removed and with output to a 19" monitor, and found that I could get wattage figures down to around 25-30 for a machine with good overall performance. I could get even lower power figures on certain laptops by employing under-clocking, even as low as 12W on a Toshiba down-clocked to 175MHz, but performance was too poor to be acceptable. A Raspberry Pi at 2-3W did well as a router stats gatherer, and still does, but it isn't up to use as a desktop.
So I investigated low-wattage chipset motherboards and eventually settled on a Gigabyte mini-ITX J1900N motherboard with Celeron quad core 2.4GHz/core inclusive - a snip at £50 from eBuyer. Add in an ITX case at £10, a 2GB stick of low voltage RAM at £10, and using a Pico-style power unit at £10, spare 2.5" SSD, and a spare 12V adapter, put together the whole thing for just £80.
In use, the machine performs really well, responsive, quiet (it comes with passive cooling), and it consumes between 9W at idle and 13.5W with all cores forced to run at 100%. Though it is targetted for use with Windows, and comes with said drivers, I'm not a Windows fan, and instead run Linux Mint, my usual favourite Linux distro - and it runs very well indeed, so much so that It is now my main usage computer. Cooling is fine; around 45C with totally passive cooling (no case fan), but I've added in a tiny 12v quiet cpu fan which results in typical temperatures of 34-36C. I'm not a 'gamer' so can't comment on its suitability for that purpose, but as a general-use desktop it suits well.
I did also install Windows 7 as a test, and it ran with good performance too.....no issues - but that was just out of interest in case anyone asked.
http://www.ebuyer.com/725330-gigabyte-ga-j1900n-d2h-celeron-2ghz-vga-hdmi-7-1-channel-audio-mini-itx-motherboard-ga-j1900n-d2hhttp://www.amazon.co.uk/dp/B00LVYF3HE/ref=pe_385721_37986871_TE_itemhttp://www.ebay.co.uk/itm/321690973400