going to change my tires, shall i be doing so to the rims also?

i have a 330ci with 17" on it (factory), and thinking of getting new ones but 18". What are yout opinions, and if you're well informed about the subject please do tell me some advan and disadvan..

thanks all

Advantage - Looks
Disadvantage - Harsher ride, more expensive tires (not to mention cost of the wheels themselves), more easily bent on big potholes, and likely heavier and weaker than stock unless you spend the money for good wheels.

Unless you spend big money for some lightweight rims and tires, this will slow you down. If you have that money, then that is great. Go for it. But if you don't, then why not spend it on some good lightweight 17's, and some Michelin PS2's or PS3's? Lightweight 17's are cheaper than 18's, for the same model wheel. And 17 tires are a LOT cheaper. My car had General's on it when I bought it, and when they finally wore out, I put on Michelin PS2's. They are a lot lighter then most tires, although I am sure there are probably a few others that are even lighter. But just that change was very noticeable to me. That convinced me to stop looking at 18's, and find some 17's that are at least as light as the OEM wheels, preferably even lighter.
It really comes down to if you want show, or go. You can have both with 18's, but it will cost you a lot more than 17's.
Also, if you have bad roads, you are a lot more likely to damage an 18in wheel, like Tangent_ said.

Originally Posted by Tangent_

Advantage - Looks
Disadvantage - Harsher ride, more expensive tires (not to mention cost of the wheels themselves), more easily bent on big potholes, and likely heavier and weaker than stock unless you spend the money for good wheels.

This.