Probably you want an eVent bivy - that will be the lightest, most breathable thing you can get. Though it is worth considering the the gore-tex bivys with the fuzzy interior have about the best reputation as far as condensation. And if you get simple sack, get it BIG - if you are stuck in there in bad weather you will want room to change clothes, look at the map, etc.
Two other thoughts - the Milesgear Uber bivy http://www.milesgear.com/
And if you decide that a bivy is going to be too confining, you might consider a BD Firstlight tent. You need a larger site than a bivy but less than your DR, and since the poles are inside the canopy you can stake out the floor, crawl in with your pack, and then set up the poles.