Im a fan of wool EVERYTHING. Nothing regulates body temperature the same. Every time I wear synthetics I get too hot or too cold, and feel as though my skin cant breathe. A good wool layering system keeps me COMFORTABLE, not just warm, at a wide range of temps. Here in Minnesota, weather can change a lot in a short time. 
Yes, Im even talking wool long underwear; there are a lot of companies doing it right now days. For boots I like good packs with wool felt (NOT thinsulate) liners, thick wool socks over wool or silk liner socks. I think the Stieger Mukluks are overpriced and if the snow melts mid day, youre in trouble. avoid gore tex footwear AT ALL COSTS for cold weather, once the cold gets in there your feet aren't getting warm again. I remember one time when my feet got cold in a pair of thinsulate and gore tex boots, I mean cold, I got in the car finnally and got the heat blasting in the foot area, my feet were still FREEZING even in a HOT car! If your concerned about wool itch or dont want to pay the admittedly high prices for good wool long underwear, go with silk instead of synthetics, your skin will thank you (and the people sitting next to you as you wont stink)
To be specific, my ideal layering for very cold (-20 F excluding windchill) would be the following, easily modified with lighter weight layers and finnally less layers as climate dictates, thats the great thing about layering:
Base: wool long underwear, 
Mid: mid weight, low bulk, terry type wool (like Ullfrotte or Icebreaker), wool shirt, vest (great for core warmth without making your sleeves too bulky), wool sweater or combo.
Outer: heavy wool bibs and jacket from someone like Filson (I like bibs for extreme cold because they let the warmth from your legs travel up to your torso and also insulate your kidneys)
Outer (packed): if its wet or windy, a good packable gore tex parka and pants are nice 
Foot: silk or lightweight wool liners with expedition weight socks (100% wool, not a blend) and good wool felt pac boots. If its milder i go for heavy wool socks with liners and good leather boots that are somewhat loose fitting to let the air circulate (VERRY important with footwear in the cold) maby with a wool felt insole.
Hands: thick removable wool liners and leather gloves, also thick wool chopper liners and large leather choppers (can also put wool glove liners and chopper liners together for extreme cold) or the King of the Mountain system is nice because its a gauntlet and covers your wrist.
Head: wool balaclava, wool stocking cap or woven wool cap with ear flaps (like a Filson or King Of the Mountain), wool neck gator or scarf. I dont like hoods because they reduce peripheral vision and also hearing.
Here are some online products and companies I like. I admit I lean more towards the expensive and high quality, it all depends on how much you can spend:
Filson, a great name in wool outerwear:
http://www.filson.com/home/index.jsp
King Of the Mountain, great if you can afford it:
http://www.kingofthemountainonline.com/wool.htm
Cabelas, search wool and also there gore tex stuff is nice (and they carry tall, hurray!)
http://www.cabelas.com/home.jsp;jsessionid=K11FDMI4BUH1OCWQNWRSCNYK0BW0IIWE?_requestid=143029
Ullfrotte, wool under and mid layer (partly synthetic, yuck) many swear by it.
http://www.shopatron.com/index/235.0
Icebreaker, wool everything! Great quality, even t-shirts and underwear, good socks and balaclavas too. Sucky thing is they dont carry my size! (curse being tall and skinny!)
http://www.icebreaker.com/site/home.html?id=htmUJrHf:71.195.6.124
Drews boots, now THATS a pac boot! (its getting hard to find 100% wool felt pac liners, I have some old ones, even here there thinsulate blend, anyone have a source?)
http://drewsboots.com/pac.htm
Here are some cloths I like form the above stores in order of layer / etc. 
Jackets:
http://www.filson.com/product/index...&cp=2069836.2069837.2075070&parentPage=family
http://www.kingofthemountainonline.com/ec/additem.asp?pid=shja04
http://www.cabelas.com/cabelas/en/t...?id=0034308960128a&type=product&cm_sz=Regular
bibs and pants (outer)
http://www.filson.com/product/index...&cp=2069836.2069840.2075076&parentPage=family
http://www.filson.com/product/index.jsp?productId=2092295&cp=2069840.2075071
http://www.filson.com/product/index...&cp=2069836.2069840.2075076&parentPage=family
http://www.kingofthemountainonline.com/ec/additem.asp?pid=pabi01
http://www.kingofthemountainonline.com/ec/additem.asp?pid=pabi02
http://www.kingofthemountainonline.com/ec/additem.asp?pid=pabi03
http://www.cabelas.com/cabelas/en/t...=mode+matchall&Nty=1&Ntt=wool+pants&noImage=0
http://www.cabelas.com/cabelas/en/t...?id=0034308960131a&type=product&cm_sz=Regular
http://www.cabelas.com/cabelas/en/t...=mode+matchall&Nty=1&Ntt=wool+pants&noImage=0
shirts and midlayer (torso)
http://www.filson.com/product/index...&cp=2069836.2069838.2075078&parentPage=family
http://www.filson.com/product/index...&cp=2069836.2069839.2075074&parentPage=family
http://www.filson.com/product/index...2&doVSearch=no&pageBucket=0&parentPage=family
http://www.kingofthemountainonline.com/ec/additem.asp?pid=shja02
http://www.cabelas.com/cabelas/en/t...de+matchall&Nty=1&Ntt=Woodsman+Wool&noImage=0
http://www.cabelas.com/cabelas/en/t...tx=mode+matchall&Nty=1&Ntt=whipcord&noImage=0
http://www.cabelas.com/cabelas/en/t...?id=0022572942019a&type=product&cm_sz=Regular
http://www.shopatron.com/product/product_id=GFB6234/235.0.19539.19543.0.0.0
http://www.shopatron.com/product/product_id=GFB6236/235.0.19539.19543.0.0.0
http://www.icebreaker.com/site/catalog/range.html?gender=Man&layer=Mid&range=Sport320
http://www.actiongear.com/cgi-bin/t...y%20pully&backto=%2Fagcatalog%2Fresults%2Etam
midlayer (legs, to wear over silk in extreme cold)
http://www.shopatron.com/product/product_id=GFB6344/235.0.19539.19543.0.0.0
base layer:
http://www.icebreaker.com/site/catalog/range.html?gender=Man&layer=Skin&range=Skin200
http://www.shopatron.com/product/product_id=GFB6342/235.0.19539.19543.0.0.0
http://www.shopatron.com/product/product_id=GFB6122/235.0.19539.19543.0.0.0
http://www.campmor.com/webapp/wcs/s...=200305066&langId=-1&parent_category_rn=61294
http://justincharles.com/store/adventure-outfitters/merino-wool-long-underwear/
head, hands and feet (only a few, you can find a lot of this stuff locally, especially socks)
http://www.icebreaker.com/cgi-bin/icebreaker/catalog/search.html?gender=Man&flag=hats_gloves_scarves
http://www.kingofthemountainonline.com/ec/additem.asp?pid=hats01
http://www.kingofthemountainonline.com/ec/additem.asp?pid=glmi04
http://www.kingofthemountainonline.com/ec/additem.asp?pid=glmi01
http://www.filson.com/product/index...&cp=2069836.2064999.2118244&parentPage=family
http://www.filson.com/family/index.jsp?categoryId=2075119&cp=2069836.2065000
the only light weight boot I know of lined with wool:
http://www.filson.com/product/index...&cp=2069836.2065000.2075093&parentPage=family
my favorite gore tex outerwear
http://www.cabelas.com/cabelas/en/t...matchall&Nty=1&Ntt=mt050+quiet+pack&noImage=0
http://www.cabelas.com/cabelas/en/t...rid=&parentType=&indexId=cat470082&hasJS=true