Looks good so far. Normally I would tell folks that a sound card isn't necessary but in your case you'll definiately want something better than the motherboard sound. Specifically make sure you find something that has all the inputs you require such as midi, etc.
For Ram I'd get 8 gigs of DDR3 1333 or 1600, there are some great deals right now.
You'll be able to add a very nice video card, and possibly a second down the road if you wish. But one is good, I'm a gamer and I've never had multi-GPU but if I could afford it I would do so. For one video card the 850W is overkill except if we're talking about a $500 card like the GTX 580.
For boot SSD I'd go with either the OCZ Vertex III or Crucial M4 120GB. Best bang for buck in this range.