I'd suggest you go for winter tyres (something like the winter 2 mentioned below) rather than a specific width. I'm not sure if people generally realise, but winter tyres have a different rubber compound which remains soft at lower temperatures and hence will have more grip than a summer tyre for the same situation. Also once you've invested in some winter tyres you can just swap back to your summer tyres when the time comes and hence don't have spend any more on tyres (although there is a greater up front cost).
I cycled through snow and ice during the last winter and bought a single winter tyre without studs (I think it was a Continental top Contact Winter 2) and I think that was a bit of a mistake as I slid on some ice and fell off
. This year I'm going to buy another conti with studs and see how that holds up.
hth
sal