Nice post Xanthe,
I am surprised even with
underclocking your 3 7950s you can get by with only a 850psu. These cards supposedly draw up to 300w under full load. Then trying to power other peripherals make it seem like your power supply would be running at 100% capacity(or more). Even simple calculators would recommend at least at 1000 for those three cards.
http://support.asus.com/powersupply.aspxAlso, When you draw at such a high percentage of the rated capacity you loose power efficiency. Supposedly running at around 50-70% gives you the best power efficiency. Above those numbers you are burning more electricity in regards to heat. Even using a high efficient power supply..like a 80plus gold.
http://en.wikipedia.org/wiki/80_PlusAnyhow, seems like you have yours dialed in to give you results and performance you are looking for.