the eVGA 780i SLi is one of the best boards you can get right now! abit more than XFX's 780i but you get much better support and the reviews on it speak for them self!
the 8800GT 512MB is what im going to get my self, best all round and better clock speeds than the GTS that cost more! Then in the future when you want to upgrade you can get a 2nd 8800, then a 3rd! the eVGA 780i also has the new CPU Technology as well so thats as future proof as you can get right now.
you will prob not need more than 200GB unless your a downloader like my self, then you should get 1TB to keep safe, but make sure your operating system is on a seperate drive. even consider RAID 0 if you want that extra performance
Stick with the on board 7.1 sound to be honist, its hi-def and usualy pritty good. will save you money and prob wont make much of a difference.