The lowest power consumption cpu you can make a great gaming computer from is probably an intel e8400 . Power consumption is only about 33 watts http://www.xbitlabs.com/articles/cpu/display/intel-wolfdale_11.html.
But alas socket 775 is just leftovers now and if you want some future proofing then
AMD's x3 720 be is a stronger gamer and has some good power saving tricks .
Unlike the intel which only clock down moderately when not under load the Phenom drops to 800 MHZ so while it might use more power in intensive use it will probably use less when you are performing less intensive tasks like reading your email
A 790 gx mb has great onboarf gfx and can even play crysis but not at high resolutions and high quality . Depending on monitor resolution have a look at amd's 4770 which is also very low power drain .
You can save about 3-4 watts with a wd green drive [ compared to wd blue , but samsungs are quieter , faster and almost as low power usage depending on model