Thanks for your answer.
Neither your cpu nor memory will bottleneck you in any way.Buy the cheapest things you can find.In fact i think your PSUs are overckill as well.I have miners with 1x6950+3x5830 with cheap CM gx650 and they are fine.
I think that for a price issue only I will go for Dual Core Athlon X2 3GHz... they are cheap enough and they are only 65W maximum TDP. So that is just fine.
About memory it will be ok too. I will go anyway with 2X2GB DDR3 1333MHz. They will cost me about 20 euros so ... not very expensive either.
About the PSU's.
Each 6990 can drain 375W not overclocked. I will not overclock any board anyway.
But not overclocked they can drain 375W.
4X 375 = 1500 W, (1.5KW
Say the rest of mobo + CPU + fans = 100W max
Total system power consumption will be = 1600W
Almost all power supplies have and efficiency curve that depends on their load. Usually the maximum efficiency is at 50% load.
My OCZ PSU's (combined 2 X 1000 W) system will operate at Maximum Efficiency of 90%. (They are rated at 92% at 50% load).
If you choose a Less Powerfull PSU you will not only be outside the maximum efficiency of your PSU's, therefore Increasing power costs for the same MH/s result, but more importantly you would be Pushing the limits of the components on those PSU's, thereby limiting there life expectancy (MTBf at higher temps) and increasing overall heat inside your box.
I think that in systems like these the PSU is paramount not only from a efficiency stand point but also for system stability and long term operation.
I can make savings on all components Except on the PSU's ...