I went last year and tHe most useful thing I'd take is a neoprene mask. The first two days had a high of 10 degrees, and its nice to have your skin covered. As long as you got something w/ 10k waterproofing and some long underwear (wal-mart brand) you'll be alright body-wise. If your jacket is just a shell maybe try a hoodie? I had a 3 in 1 jacket last year and it worked fine.
Big sky is fuckin killer. he blacks on Andesite mountain are the gem there if there is some fresh pow. get there early and happy hunting.
'09 Flow Infinite 164w + '09 Flow M11's
'09 Capita Mid Life Lost 158 + '09 Rome 390's
'08 DC Graphix BOA's