No contest...
4x 7850:
Max Power Draw = 4*130W = 520 W
Approx LTC Hashrate = 4*330 kH/s = 1.32 MH/s
Kilohashes/Watt: 2.54
Upfront Cost = 4*$190 = $760
2x 7950:
Max Power Draw = 2*180W = 360 W
Approx LTC Hashrate = 2*620 kH/s = 1.24 MH/s
Kilohashes/Watt: 3.44
Upfront Cost = 2*$300 = $600
2x 7970:
Max Power Draw = 2*270W = 540 W
Approx LTC Hashrate = 2*725 kH/s = 1.5 MH/s
Kilohashes/Watt: 2.78
Upfront Cost = 2*$420 = $840
The 7950s are typically all but impossible to beat as a mining card for any non-BTC mining.
In this example, the two 7950s have the cheapest upfront cost, consume the least power, and trounce the other options in terms of hashes per watt.
It's my understanding that for mining, the Pitcairn-based boards are not the best option; for a mining rig, consider only Tahiti-based cards (7870XT or higher).
That said, the 7950 is almost always the best bet, regardless of the number of cards. Hope this helps...
Every single thing in your 7850 calculation is wrong. Brand new 7850's are $140 at newegg, mine hash at ~375 on a pretty mild overclock (and still run at <60*C at 75% fan), and draw slightly more than 110w each. I'm running two sets of four on 800w power supplies and haven't had a single issue in the three months I've had them. During the Never Settle deal they were actually only ~$120 after you sold the codes.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814202004
