I think that an antminer S5 is you best option because the have relatively low power consumption and they produce low noise.
Spondoolies SP20 is also a good option but the fan is loud according to reviews.
I personally have a fishtank and I intend to buy some prisma blades, just the blades and immerse them in cooling fluid. It's the cheapest and quietest option.
the s-5 is crazy loud worse then the sp20
Change the fan and the noise would go away. They ship with really bad fans.
I actually think the op should look for 2 s-3's he has free power.
2 s-3's clocked to freq 200 are really quiet and get 800gh using about 640 watts he could run two on his 850 watt psu. and some s-3's cost about 150 or less on ebay
http://www.ebay.com/sch/i.html?_from=R40%7CR40&_sacat=0&LH_BIN=1&_sop=16&_mPrRngCbx=1&_udlo=100&_udhi=139&_nkw=antminer%20s-3&_dcat=179171&rt=nc&_pppn=r1he can buy some at 138 or less 2x 138 = 276 he has a psu. so he can do 2 clocked at freq 200 pull 600 watts maybe 640 watts pretty quiet. 800 gh earns him about 280 + 40 by june so he would pay the 280 off and make 40 bucks with very safe numbers. for the estimate.