One word. Layer, layer, layer. Okay, that's one word three times, but you get the picture. Maybe even moderately hefty layers—Mount Washington in January is potentially a pretty nasty place. After all, it has an official record low of -50 F, and once recorded a wind gust of 231 miles per hour. That is not a typo. Two. Three. One.
I'd start with a good, light, wicking T. Mountain Hardwear's long-sleeved Wicked Lite Double T ($40) is a good one. It provides some light insulation next to the skin, but more importantly keeps you dry. Over that, a warm base layer. I'd recommend wool, such as the Icebreaker BodyFit 200 Mondo Zip-T ($80). Then another, heavier insulating layer. The North Face's Pumore Fleece Jacket ($99) is a classic piece. It's made with Polartec 200, and feels light yet offers a surprising amount of warmth.
Over all of that, toss a good shell. I currently like REI's Shuksan ($299) because of its great price and use of highly breathable eVent fabric. Arc'Teryx's Theta AR ($500) is rather pricey, but comes in a longer cut for more coverage and uses Gore's excellent Pro Shell material. It's a totally bomb-proof shell.
That would be my basic kit—and there are equivalent pieces for your legs (although there you probably could get by with either a wool or fleece layer and a shell over that). But I'd carry a few other things. Some sort of fill-insulated piece, for instance. Maybe Patagonia's Nano Puff Pullover ($149) because it would layer nicely under a shell, yet add a lot of warmth. Hat and gloves, obviously. And something for an emergency—your basic ten essentials, of course, plus some sort of desperation shelter, such as a Space Emergency Bag ($10).