The Haswell platform is limited to 32GB which makes the X79 the choice for you. The 780GTX is the best combination of performance and price. I would suggest that and spend the savings on an SSD drive. I would also suggest getting a Firewire card so the board options are open. The best boards configurations often do not have built in Firewire. The 3930K is the best CPU for the price and easily clockable up to 4.5GHz. What audio interface are you using with Nuendo and I highly suggest a Blackmagic card if you are using Nuendo since that is supported.
That limitation on the 4770K (Hasswell) is now clear for me.
On the Intel website, the i7 Extreme Processors list, the 3930K is not present. It's a 2nd Generation.
Should I opt for it of for a 4th Generation ?
On the audio side I will check with Steinberg.
The Matrox MXO2 has 5.1 audio monitoring. I will check if it works with NUENDO. With Audition CC it should work for sure.
Only the Desktop platform has updated to Gen 4. That is limited to Quad Core and 32GB of ram. The workstation X79 platform is Gen 2 right now. That is the option with 6 Cores and 64GB of ram.
I know Nuendo works with Blackmagic and that is what most Nuendo film users have. I dont believe I have ever tested Matrox with it.I would suggest you go with what is certified with Nuendo. Both will work with Audition.