I've always tried to ensure that I've had all weather (or winter - there's a difference) tyres fitted in time for the crunchy stuff in the winter; usually I've ended up with two 'in the nick of time' and all too frequently I've not had the spare cash to have a spare set of wheels with winter tyres on just lying around.
From plenty of experience in upper-don and deeside I can honestly say that you can
just get away with two 'winter' tyres on the front (in a front wheel drive car...) and if you're set up that way the chances are you're better prepared than most. However, that experience also lets me say that the first time you get into serious snow (let's leave ice out of this) the back end of the car will get lively and especially so in the bends - hugely differing grip levels between summer and winter tyres make for some interesting corner techniques
Left-foot braking is a useful trick to learn, and
quickly This winter I've bought a full set of spare 15" alloys (ebay - wonderful place) and a set of these
http://www.conti-online.com/generator/www/de/en/gislaved/tires/themes/winter-tires/eurofrost-3/eurofrost-3.html which I've been told are the mutts nuts come the bad weather. They only went on this evening, so time will tell how they'll shape up but they look damn 'chunky'.
On reflection, some are better than none but four are better than two
Total investment - tyres and rims - under £400. Money well spent
H
PS - Ice...... now that's another story