Well its still a little tough to find E6600's, especially at SRP. Where as the 6300 and 6400's are in stock at Tankguys at good prices. If you do OC, there is no reason why one of these won't give plenty of performance for what its going to be used for.
I don't think you can pass up getting a E6300 for $200. Usually I even say go for the 6400, but since really 2.4-2.8 should be twice the power your actually going to need there is no reason to spend it on the CPU. The money is better spent somewhere else.
You could even possibly get away with the Asrock MB. Extremely cheap, will do a little OC'ing, and its got quality components from what I've read. Otherwise I think you have to go with a DS3. The P5B standard is just not worth it and moving up to $200+ for the MB I think is to much.
I'd get the seagate 320g 7200.10 for $95. If your doing lots of movie backup, 2 may even be a good idea. 600gigs gets used up fast these days, atleast for me it does.
Ram you can be fairly loose with. Since you prolly won't OC past 400 mhz, any DDR2 800 or quality 667 will do the job. Just have to be careful and make sure someone has reported it will boot with the board you get.
GPU - Get the best you can with the money you have left over. If you got $200 left, get an X1800xt.
You still have to cover the PSU. Something along these lines:
http://www.newegg.com/Product/Product.asp?Item=N82E16817103488
