I second what the other guy said. I train year round in New England, and I've had a crash course in dressing for the elements. I have a pair of goretex-over-neoprene tights that are good down to about 20 degrees. Colder and I wear tights underneath and I'm good down to around zero degrees F.

The feet are a weak spot because most booties don't cover the sole, and also the pedal/cleat is a heat sink, so you lose a TON of heat through the bottom of your foot. A nice thick wool sock helps, but so does a foam insert.

A wind-shell is also key. Mine is a Canari that has retro-reflective piping and zip-off sleeves. It works fine.

Also, get fenders. If you've never ridden in slush/rain with them, the difference is huge. My winter bike is a single speed rigid MTB with PlanetBike polycarbonate fenders (plastic ones buckle and crack in extreme cold), and they are awesome.

A hat you can wear under your helmet is key, and a face mask is a good investment. Mine is a $15 neoprene jobbie from a ski shop, and I cut out the mouth area for drinking and breathing.

Dealing with the cold is a learning process. Check out icebike.com for a ton of good info.

GB