Hey! That was my next build idea!
I think the case is a great choice, and has plenty of water-cooling possibilities. I would choose the DS3 over the P5E anytime - one heck overclocker! My vc choice was the 3870 - with the hot air exhaust to the outside this will help the internal temps. As for the rad, I was looking at a dual 120 on one side.
Which PSU have you chosen? Modular, I hope?
Have fun building it, it will look awesome!