Unless you pay H U G E money for electricity, I wouldn't care much about efficiency.
Using this CPU calculator and just picking a 920, ya get a system load of 143 watts
Using this CPU calculator and just picking a 3770k, ya get a system load of 100 wattshttp://extreme.outervision.com/PSUEngine
But IIRC, he 9xx series came out in like November 2008 .... over 3.5 years ago .... that's an antique in PC terms.
If you can get it for under $500, it would be an OK buy..... if not, I'd take $1200 and build something better. Water cooling gave ya something "back in the day" but since SB/IB, it really serves no purpose except for the peeps who wanna get their names on OC lists.....not that that isn't a fun thing to do in and of itself just to see if ya can do it.