First off, at full load, even if you OC, you won't need any more than a 600W PSU, if you want to future-proof, you could get a 700-850W PSU, but 1200W!!! Come on, that's ridiculas!
320GB Western Digital HHDs sell for $75 - that's 4.27GB/$
400GB Western Digital HDDs sell for $95 - that's 4.21GB/$
500GB Western Digital HDDs sell for $110 - that's 4.54GB/$
320GB Seagate HDDs sell for $80 - that's 4.00GB/$
400GB Seagate HDDs sell for $100 - that's 4.00GB/$
500GB Seagate HDDs sell for $120 - that's 4.17GB/$
320GB Samsung HDDs sell for $80 - that's 4.00GB/$
400GB Samsung HDDs sell for $85 - that's 4.71GB/$
500GB Samsung HDDs sell for $110 - that's 4.54GB/$
In the case of the Western Digitals and the Seagates, the 500GB drives offer the best value, the Samsungs offer better value from their 400GB SpinPoint T drive at $85. It's also known for being very quiet. So I'd reccommend switching to those (plus it'll give you 800GB of usable space, instead of only 640GB). Video editing is a space hog, so the more space the better.
I'd also say something about your choice of RAID 0+1. But, I'm guessing you already compaired the advantages/disadvantages of RAID 5?
Lastly,
check out the charts on this page showing coolers of varying performance. The Vigor is definitly good, but the Thermalright Ultra Extreme is as good and better, and it's quite a bit cheaper, that'd be my reccommendation. Plus the Vigor is pretty noisey.
Also consider a quad core. The Q6600 is about the same price as the E6850, but will give you a lot more power for video editing and encoding. Most shops are now stocking the G0 stepping, which are known to easily OC to over 3.0GHz (the stock speed of the E6850).
As far as a choice for an OS. I agree that Vista x64 is still a little buggy, but it'd still be my choice for a video editing rig. If you're comfortable with Linux (I am not, so I don't have any specific reccommendations), I'd also look into those options for better stability.