Well, I personally run 1000W Goldflower, because two xeons and small GPU only take up to 450+/-W, so it runs completely passive. I am noise-freak.
Each Xeon (with exception of high clock/turbo WS models) are 115W (or bit more if they turbo all cores), GTX780 is 250W under full load, the rest is meaningless, count 50W for convenience.
So Dual-Xeon with active-PSU and single 780 would draw 230+250+50=530W in average, so if you like proofing, 650W, or possibility of second GPU, you can go up to 1000W. Bot not 1200W :- )
Yes, you need 1600Mhz. Even though Asus officially states 1866 support, I've seen people struggle to get it, mine even had problems with 1600 heh....on latest bios.
Bellow 2660 are not worth considering. They are too low clocked to be worth the overal PC cost. 2660v2/2670v2 are the low budget option (should be able to get street price of around 1200-1400Euros for each),
2667v2/2680v2 are middle ground (1500-1600) euros for each, and workstation 2687WS is highest you should go, it's quite expensive with only little benefit from single-thread performance.
Don't really consider the rest.
It's worth contacting custom builders, they have access to direct sellers (B2B, not consumer), and can ask around which wholesale sellers can order it.
The total cost for full workstation, without GPU(you already have one spare), with middle choice (2667 in preference for singlethread/2680in preference for multithread), with high-end components (platinum 1000W PSU Goldflower/Seasonic, Fractal XL2, Noiseblocker 140mm case fans) should not exceed 5000 euros in total cost with VAT. If you are direct VAT payer, than -20perc. so 4000 euros.