CPU-Intel i5 2500k
MOTHERBOARD-ASRock Z77 Extreme4
GPU-Sapphire Radeon 6870/ Maybe x2 in future
RAM-Corsair Vengance 8GB
CASE-NZXT Phantom case
PSU-Powercool Modular 850w 80
and a 1tb hard drive and a dvd burner...
Welcome to Tom's Hardware Forums,
All of the parts you show are compatible but you have a weak link in your build - the PSU. I can find no objective reviews on any Powercool PSU, I found a few threads saying they had problems but nothing real good. I will personally suggest you steer clear of any PSU which has not been reviewed - see this list http://www.realhardtechx.com/index_archivos/Page541.htm
shows where to find reviews for PSU's (by maker)
Also you might spend some time reading this about PSU's. http://www.tomshardware.com/forum/324368-28-computer-po...
Happy reading, enjoy your build (with different PSU)
May go with your option but I would like to future proof this PC and make sure I have nothing to worry about when installing more power hungry parts.
Power hungry parts are becoming a thing of the past. The new generation of GPUs are far more energy efficient than previous generations so the days of 1K+ power supplies are coming to an end. A 950W will handle 3 x SLI for the GTX 670 easily.