Can't emphasise enough on BOOTS, BOOTS, BOOTS!!! Don't scrimp on them, spend as much as you can afford, as once your feet get wet, they stay wet, and then they get cold, and then you just want to go home.
Under armour type clothing is great, rather than thick layers, these are good at keeping you warm and retaining flexibity.
Hat or what I wear is a north face ear protecter, which keeps my lugs warm, but doesn't get me too hot.
Gloves...lots of debates on these in here already...
Oh, and a hot water system.