Dual Xeon's for twice the goodness.
Your talking high end so you will likely want more video than the 2600k provides. It is very good, but you can do better
With the 990x you limited to one CPU
Dual Xeon you could get mor extreame with the x5690 as the best all around choice.
Processor Number X5690 X5680 # of Cores 6 6 # of Threads 12 12 Clock Speed 3.46 GHz 3.33 GHz Max Turbo Frequency 3.73 GHz 3.6 GHz Cache 12 MB Intel® Smart Cache 12 MB Intel® Smart Cache Bus/Core Ratio 26 25
If the software he uses is single threeded you may want to consider a
Processor Number X5687 # of Cores 4 # of Threads 8 Clock Speed 3.6 GHz Max Turbo Frequency 3.86 GHz Cache 12 MB Intel® Smart Cache Bus/Core Ratio 27
or a x5698 (if you could find one) http://www.cpu-world.com/CPUs/Xeon/Intel-Xeon%20X5698%20-%20AT80614007314AA.html
If your planning to overclock the board the SR-2 is about the only way to go. (make sure you have some real good H20 cooled heatsinks.)
If your OK with Intels settings and speed. you may want to check out a S5520SCR mother board or a http://www.intel.com/content/www/us/en/server-systems/server-system-sc5650scws.html?wapkw=intel workstation system sc5650scws system
So I will go with dual xeons then