I am planning costs for my rigs, and I am kinda confused about Your post. If 4 cards = 1200W, 2 cards = 600W. Why You said 850W??
2 cards = 600W, .............+ MB + CPU + HDD, etc = 800W
so then 4 cards = 1000W + + MB + CPU + HDD, etc = 1200W
250W per card, so
2cards = 500W + rest 700W
