You wont need six cores till you're into serious (read pro level) encoding anyways.
The rest of your stuff looks really good. Looks like you've done your research
But mosox is right 'bout the 6000 series GPU's. If you wanna go with ATI, might as well wait a while.
Actually now that I think 'bout it, since you'll wait 3 weeks anyhow, why don't you wait another few for the Sandy/Bulldozers to come out, that'll reduce the prices a bit, IMO. Also, raising your budget a bit might then get you the brand new stuff!