In reply to Rav2: The humble sleeping mat is the most important item when it comes to winter camping. Don't be cheap when it come to spending either cash or grammes on one. You'd be surprised at what you can get away with sleeping bag wise if your mat insulates you properly and just how cold you can be in a decent bag when the mat is wrong.
Lot's of people use single pole tents even in winter, myself included, and never have any issues - if you pitch well and in the right place, most decent tents can cope. A geodesic is stronger but way heavier, how much weight are you happy to carry?
Best tip I can offer - eat well just before you go to sleep and do some star jumps or something before getting into your bag. You'll be warmer for it later.
Have fun!