i'm telling you , you have the best winter setup in terms of wheels for this car (the stock 18's) buy some snow tires and upgrade or put back on the summer tires in spring.
found these on craigslist if you wantto inquire
http://newhaven.craigslist.org/pts/2057789089.html
http://longisland.craigslist.org/pts/2060143190.html
http://longisland.craigslist.org/pts/2032185269.html
these you could buy the 245/45/18's for 100 each and then get a second pair from tire rack for full price and make out ok $ wise
http://newjersey.craigslist.org/pts/2086374843.html
tire sizes you can go for on the stock 18's are 245/45/18 all around, or 245/45/18 in rear and 225/50/18 in front.