Case is going to be a tight fit for twin GFX. You have all hi end components and with a 5-6 year plan to keep the box, I'd go bigger.
Antec 1200 is $60 more but has great cooling and will last many years. Paired with the CP-850 PSU, the total is $250....but both will give you a solid platform for twin GFX and OC'ing. As an alternate, consider the HAF-932 / HX-850.
For performance and ease of installation, the Prolimatech Megahalems fits both criteria. Here's what I'm putting in new builds Mega w/ IC Diamond TIM and twin Scythe PWMfans (make sure ya MoBo can handle the fan wattage). ($95 for the HS, TIM. two PWM fans and a Y cable splitter))
Thank you so much, I'll probably take your advice on the Antec 900 + power supply.
I have a couple questions though - Would I need a heat sink if I did not plan to OC? How important is it?
Also do you have any idea what GPU would fit this system best? From what I've seen what I listed seems appropriate but I don't want to overkill it and waste money or bottleneck the other high end components...
Stock heatsink is OK if you will not be OC'ing. But... I would suggest an oem heat sink anyway. A tower type with a large 120mm fan will keep your cpu cooler and make less noise when under load. It is better to install one up front since it needs to be done outside of the case. I hate to uninstall a motherboard in a working PC later. If you OC, (and it is very easy to do with a i7-920), then a good cooler will let you go higher and easier. A decent one should cost <$50.
Parts look good, except possibly the hard drives. A single 1tb drive is cheaper, and will perform better in most cases. If you are planning on two drives for raid-0, then don't bother; it is not worth it for gaming.
The stock HSF on the 920/930 is barely adequate. It's not like the i5-750. These CPUs run hot. Even if you do not OC, I would personally spend the ~$30 on the Cooler Master Hype 212+. If you do want to OC, then I would not skimp on the HSF. Someone above mentioned the Megahalems. That's a good one, but I'd personally buy the Noctua NH-D14, which outside of a WC setup is untouched on a 4.0+GHz 930.