Regarding GTX 770 and possibility of two. 
I am running an early ASUS MB running an old i7 940 CPU.   I run two ASUS ATX 770 DirectCU II which I purchased about a month apart  in early 2014. I OC the CPU, memory, and both GPUs--all air no water.  
I found that my two GPUs have significantly different profiles. One is notably faster than the other.  My biggest issue in tuning was to make sure I have a GPU tweak that would tune and remember parameters for the two devices individually.  Over time I have been able to tune them so that they both operate reliably just below the performance at which they begin to fail. 
I run 6 of the available 8 "CPU" for folding.  I generally see no degradation in the user experience. 
I also run another slow ATX card in a slow Dell off the shelf system. That was all of the slots and power available in that system.  
The system with the two 770s runs F@H  pretty much 24/7 365. 
From January 2014 to today, I have been able to go from 300s point to  more than 41,000,000.  F@H says I am user 1827 of 1,752,412 at the moment. 
Conclusion--770s were just before the knee in the cost curve when I bought them, and they are working well 1 year later. 
Good luck.
			
			
									
						
										
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