Well, after looking long and hard, there aren't a lot of choices. I'm sticking with 17", it will be a bit softer and have more grip on the road, plus the tires are slightly cheaper than 18"

Since its winter too, with all the salt, I'm going with replica rims too, OEM just isn't worth it for the salt and slush of winter. I'm going for the CSL replicas (OEM Style 127 in 17") as rims. They are the YKW Motorsport BMR-3 17x8 5x120 ET35 (ET40 is better but they are sold out) YKW Motor Sports Inc. - EURO Klasse

For tires, I'm sticking with a set of 4 Michelin X-Ice Xi2 235/55R17 - I believe you get what you paid for with tires. Very highly rated - but I live in Toronto, Canada, so I might get more snow and slush than you.

I haven't quite seen an X3 with CSL rims on before, but they will look OEM since they accept the OEM BMW center caps. I'll let you know how they look when I get them on this weekend!