Hi,
get a mummy bag. I like them cut large; but that adds some weight. There should be a lot of bags going on sale soon. I have had good luck with LL Bean stuff, this is just an example of a decent bag http://www.llbean.com/webapp/wcs/sto...Id=1&langId=-1
I love this one
http://www.campmor.com/webapp/commer...22&prmenbr=226
Here's another wide one from a good company
http://www.campmor.com/webapp/commer...40&prmenbr=226

Having spent a few miserable nights; my habit is to get a bag rated 10 or even 20 degrees cooler than what I am expecting. First night I spent on a mountain
the temp dropped into the 30's and the wind howled. This was in August.