You really don't want to go coilover. The ride will be very hard and rough.
I live in Philly, if we get 12+ inches in one snowfall the city shuts down. Last year we had 2 big snowstorms one was 28 inches and the other was 18. My Alero on KYB GR2 and Eibachs was just fine. The look on my nieghbor's face after pulling out of 2+ feet of snow and being plowed in, while he was digging out his Land Rover, with no problems (4 back ups and go) was priceless
Of course I have pretty good kick ass all season tires on the car too, that helps a lot.
After about 16 inches, the car likes to think it's RWD. As long as your ready for that, your fine
The roads here are pretty crappy, any lower would be a problem