What exactly would the system be used for? I see a high-end GPU and only 8gb of RAM that suggests gaming, but I also see a multi-threaded purpose CPU that suggests heavy workloads like video rendering.
Do not get the 4GB 670 card. The extra memory will not help unless you have a triple display 1920X1080X3. And if you did, the single 670 won't have enough power to move that much memory fast. In short, for a single 1920X1200 display or less, 2GB is plenty.