I try and plan so that if I'm wearing everything I brought on the trip I'll sleep comfortably and have a several degrees to spare. I haven't had a chance to take my Ghost bellow about 30-32 F (-1 to 0 C or so) but have been more than comfortable every time.
As far as headwear I tried to go multipurpose. I bought a smartwool shadows hoody so that my base layer essentially seals me up. I switched to a windshirt w/a hood too. As soon as the cocoon pullovers w/hoods are availabe I'm going to get one of those and ditch my lightweight fleece beanie all together. In fact unless temps are very near 0 C or less I usually pull the beanie off in the night.
So my sleep system goes something like this: light weight wool base layer(w/hood), pertex windshirt (w/hood), micropuff pullover, gore-tex paclite jacket (w/hood). On the bottom I wear some synthetic tights and my light nylon hiking pants. I wear possum down gloves, a 200 wt beanie, and smartwool hiker socks to bed (an extra pair I changed into when I get to camp). All that under the Ghost quilt inside a silnylon and epic bivy under a tarp.
The make or break for me is insulation between me and the ground. I always make a big bed of leaves or pine needles to sleep on. I use a torso sized GG nightlite pad and put my golite jam under my legs. Recently I started bringing a slightly shortened GG thinlite to add under my legs. Its an ounce and a half more maybe but I seem to sleep much much warmer. I roll it up and keep it tied to the bottom of my bag and use it as a sit pad on breaks and in camp. And I never leave home w/out my flexair pillow. That thing has increased my enjoyment of the wilderness more than any other piece of gear.
On my last trip lows were down in the mid 30's, wet, and pretty windy. I went to bed with my arms outside my quilt and never had to tighten the straps down around me. Slept very, very warm (almost too warm) with the above set up. I think I easily could have gone to the mide or lower 20's and still been fine.