thinking about 20", 21", 22" wheels to replace my stock 18's, I see that
 

vehicles that come from factory with 21's have something called the wheel arch extensions, is that necessary for wheels bigger then 20" ?

Yes the wheel arch extensions are needed for 21" + wheels. Unless you have the air suspension I wouldn't go over 20" especially in New York.

thanks, ill probably just get a set of the 20" turbo II replicas

Here are some pictures of my wife's new Cayenne on 20" Victor Zehn wheels. Living in the Northeast, I refused to go over 20 as this is a daily driver.

those look nice, what size tires are you running ? i was thinking about 275/45 on the 20x9.5

The tires are 275/45/20 Yokohama Parada Spec X and the Victor wheels are 20x10.

nice, i was looking at the same tires, thanks for the quick responses.