If you dont plan on adding another GTX 580 then the 850 will be plenty and really even if you added a second card as long as you dont heavily overclock the system, and even if you do then 850 should be plenty. But the extra 30 dollars after the mail in rebate, would be well spent for the 1050 for future upgrades. Both would suffice but I would personally go with the 1050.
About 580 SLI power consumption from Guru3d:
System in IDLE = 237W
System Wattage with GPUs in FULL Stress = 719W
Difference (GPU load) = 482W
Add average IDLE wattage ~ 20W x2
Subjective obtained GPU power consumption = ~ 522 Watts
Both fit the CPX form factor CP-1000 and the pairing works extremely well together for low noise, excellent cooling.
"Modular" is a useless buzzword. What is the sense in "modularizing" every single cable ? Can you build a box w/o the 24 in cable ? Then why introduce a connector which is a potential failure point ? Hybrid modular combines the best of both worlds with hard wiring of the absolutely necessary cables and modular cables for the "optional" ones.
CPU - I'd suggest the 2500k fpr the $100 savings.....unless you need hyperthreading for video editing, CAD or some other work station level app, the 2500k will give you identical performance in gaming and run about 7C cooler.
MoBo - Take that money and invest in a MoBo manufacturer that offers more than a 2 year warranty. And if ya think ya need Z68 (I don't see it), you might as well get GEN3
Even for the most conservative, I would suggest OC'ing from the get go.....OC'ing has gone from something Intel winced about to something they actively encourage and have made it an integral part of their marketing strategy. A moderate 4.5 GHz OC on a 2500k will barely warm it up.
From techPowerUp.com's review of NVIDIA GeForce GTX 580 SLI the following power consumption graphs are for the graphics card only:
Idle: Windows Vista Aero sitting at the desktop (1280x1024 32-bit) all windows closed, drivers installed. Card left to warm up in idle until power draw is stable.
Average: 3DMark03 Nature at 1280x1024, 6xAA, 16xAF. This results in the highest power consumption. Average of all readings (12 per second) while the test was rendering (no title screen).
Peak: 3DMark03 Nature at 1280x1024, 6xAA, 16xAF. Highest single reading during the test.
Maximum: Furmark Stability Test at 1280x1024, 0xAA. This results in a very high non-game power consumption that can typically be reached only with stress testing applications. Card left running stress test until power draw converged to a stable value.
To handle the worse case power consumption scenario (i.e. Maximum Card Power (w/o limiter) of 627 Watts), NVIDIA recommends that the power supply have a minimum combined +12 Volt continuous current rating of 62 Amps or greater and with at least two 6-pin and two 8-pin PCI Express Supplementary Power Connectors.
It is the power supply's minimum combined +12 Volt continuous current rating specification that is the most critical requirement that must be met not its wattage.
The Corsair Professional Series HX850W PSU, that the OP linked to, with its minimum +12 Volt continuous current rating of 70 Amps meets the requirement.