Definitely go for anything other then asus for mobos. Asus is having trouble making mobos for S939s. I would definitely recommend the Gigabyte one but it's like 50+ dollars then other ones. If you have the extra cash, for that that big boi, other then that go for msi. Oh yeah, definitely get a S939 athlon 64 cpu. 3700+, 3800+, 3500+ anything you want.
Just like nubius said, definitely get the 200 dollar more cheaper 6800GT and get a better speed then 6800ULTRA with not much risk. IT'S 200 DOLLARS CHEAPER. and btw. it's pretty hard to get 6800 ultras or x800 xt pe.
sound card. hmm.. if you're really sensitive to sound and sound quality, get some audigy sound card. But if all you care is hearing sound coming out of the computer, just use the onboard soundcard on your motherboard.
Definitely XMS corsair. it's well worth it.