There's a BIG difference between 3GB and 6GB in terms of
use on a day-to-day system though. 3GB won't slow things down much, but 6GB will speed things up. SuperFetch will just gobble that extra RAM up and things will whiz along for fun. It's one of the reasons I've got 12GB (though if I was buying it, I'd have been quite happy with 6GB!).
As to 6GB or 12GB that depends entirely on what you're doing and how big the files are that you'll be playing with. If you're working with ultra-high res scans or something then you might want that extra, but it's very rare that there is a
need for that much.
If you've no intention of overclocking, just get the i7 920 or 940. The 940 isn't that much quicker than the 920, but it's a lot less expensive than the 965. It's not worth the extra over the 920, but if the CPU is important, then consider it. Most would just get the 920 and overclock it though.
As for everything else... just what others have said. 1000W overkill, consider a decent SSD or 2, etc.