what model 300 is it? IIRC the 300C and perhaps the touring that come standard with 18's won't fit the 17" rims.
and aspect ratio doesn't have anything to do with width....
anyhoo, the 2nd rims, if you want a good, inexpensive snow, look at cooper weathermaster ST2's. go 4 snows, the fronts steer and do 70% of the braking, so it's important they have good grip, too.
if you need new all seasons, look at nokians
http://www.nokiantires.com/their all seasons (WGR2, etc) are as good in the snow as a studless dedicated snow....