Gotta agree with most of what's been said. Unless I think it's going to get really cold, I make due with a merino base layer, Railrider Weather Pants, a light long-sleeve shirt (the one available right here at BPL), and perhaps a windstopper vest or the MontBell Ex Light Down Jacket. At only 5.6 oz., it's the best warmth/ weight bargain available, and it's great to supplement my Nunatak Backcountry Blanket at night.
The critical issues (for me, at least at my age) are the fingers and toes, not the torso. In cold weather, I wear woolen toe socks underneath Gortex oversocks. Gotta keep the tootsies warm and dry!
Also, I can't skimp on gloves. I wear silk glove liners, followed by possum-down gloves from BPL), followed by "windstopper," water-resistent gloves over the top. The weight cost is minimal, but earlier experiences with frostbite in my misspent youth (observing with telescopes all night in very cold weather) have left me prone to serious pains in the extremities.
Thus, please heed my advice: Protect your fingers and toes at minimal weight cost. You'll be glad you did later.
Also, carry along a sufficient sleep system to keep you warm at night. If you start out cold (especially in the fingers and toes, but not exclusively), you'll stay cold all day. I often wear my possum-down gloves and socks to bed for that reason.
And don't forget a hat that covers your ears as well as the top of your head. A hat makes up for the lack of a heavier outer layer for the usual cliched reason: You lose most of your heat through the top of your head. Furthermore, your ears are as much of an extremity as your toes.
Finally, keep your feet dry. Cold, wet feet are a recipe for frostbite and hiking misery.
From someone who learned the hard way,
P.S. Why didn't I wear heavy gloves while observing, you ask? There's nothing like a $300 eyepiece hitting the hard ground to remind you of how broke you are from spending all that cash on telescope equipment. Thus, I cut the finger tips off of heavy gloves and wore the silk glove liners underneath. Of course, the finger tips were exactly the part of the fingers I should have been protecting. Stupid.