Also, a word of caution on the Target/Walmart/whatever $200-300 models - they tend to be VERY heavy with really cheap components and tires.
A friend bought a Mongoose (53lbs, $250) just for grins and I tried it - the mechanical disc brakes were horrible (they certainly make better, but apparently not on a $250 fatty), the shifting was annoyingly sluggish, and the extra weight was very noticeable.
He busted the chain on his first off-road ride. He also said the tires weren't that great in the snow (I rode it in Summer). You want it for riding in the snow, and tires can make a huge difference, so that's one more thing to look for when shopping. But also keep in mind that tires are an easy upgrade, I've seen pairs of decent 4.5"x26 tires on ebay for under $100.