the best thing to do is to go to tirerack.com, they can set you up with their winter tire packages. the biggest thing you would have to worry about is your brakes. if you have big brakes you may not be able to run 15" wheels. 16" wheels should work, unless the brakes are huge.
You will notice radical detrimental effects, no question about it. Especially when new, snow tires are extremely greasy feeling, and have much less directional stability than summer/all season tires.
On my own (E30) BMW I went from 195/65-14 to 185/70-14 and the difference was enormous.
However, there is good news:
- The snow tires will feel much better when they have worn in a bit.
- I like the fact that with snow tires, I can drive the car hard enough to play around with it, routinely approach traction limits - and still be going at a reasonable pace. Every drive down a twisty road becomes a lesson in driving dynamics and one big amusement park ride. I like it that I get to play like that for half the year, then get the "stuck to the road" feel of summer tires for the other half.
- Nothing else compares when you're out in the snowy nastiness. With rwd I can run circles around almost anything out there.
Having used many different snow tires I now give Nokian products my highest recommendation. The traction is comparable to other premium snow tires, but the breakaway characteristics are extraordinarily predictable, and the are less disturbed by changes in traction over slush, snow, packed snow, ice and so on. I'm using Hak IIs, though I've heard great things about the rest of the range as well.