tsl is absolutely right about the snow. I have Nokian extremes front and back on a heavy rigid mountain bike. I still get 'pushed around' by the surface conditions sometimes. I'm pretty convinced they're the best tire I could be using, but nothing will make riding on snow easy. I'm never in danger of falling over, and since not getting squished by a car is my #1 priority, I consider the extra cost worthwhile. Saving a hundred dollars or so just isn't worth risking serious injury or even death. Even saving myself from a sprained wrist or bruised knee is worth a hundred bucks in my books, actually.

My first winter I wasn't sure I'd stick with it, or that studs helped much. So, I used a lightly studded 700C tire on front, and a slick tire on the back (for the $ savings). I learned my snow skills pretty quickly that year! I've since decided that I deserve to be safe, and prefer stability and a good resistance workout over speed. I still use that set-up sometimes, though, when I lend my 'good' bike and go on a ride with a first time winter cyclist, or on those rare days in fall and spring when there's just a chance of ice here and there, but none that's obvious.

Having used both a 'minimalist' winter tire set up, and the 'ultimate' (apart from getting a Pugsley, I guess), and also having ridden slicks and knobbies without studs on ice, I can tell you that there's a big difference. Get the two best tires you're willing to shell out for, and enjoy them. The extra confidence makes winter riding a lot of fun.