Good answers so far, here is my input. I have ridden in everything from 110 to -10 so I will just give the full run down based on my experience.
55 and up I will wear shorts and wicking shirt
50 - 55 add
leg warmers and t-shirt over wicking shirt.
45 - 50 add
Arm warmers
40 - 45 add Sleeveless fleece vest
35 - 40 add goretex shell jacket with pit zips
30 - 35 add head sweats head band
30 and under add goretex shell pants
If it gets lower then about 10 I will usually add a face mask as well.
I find a light shell that is windproof with pit zips to be the most important piece of clothing. You generate your own warmth as you ride and don't need to dress as warmly as you think. You want to be chilly when you start but not cold, if you plan on any stops along the way make sure you have something warm to throw on, wool sweaters work great.
Very important, make sure you keep the knees warm for anything below 55, they will ache on you if you don't.