Narrower is better in principle because narrow tires are better at cutting through rather than riding over snow, but 10 or 20mm is probably negligible compared to the difference between snow tires and all-seasons.
I think P175/65R14 (the LX stock size) is the narrowest suitable tire that manufacturers actually make. (If there are 13" rims that will fit over the brakes then P155/80R13 would be ideal.) In 15" either P195/55R15 (NA Sport stock) or P185/55R15 (Sport stock in the rest of the world, but probably hard to get here) would do.
If you are looking at tires in some other size, there is a good calculator at
http://www.carbibles.com/tyre_bible.html about 3/5 of the way down. There's also a section on speed ratings near the top.