I want an aggressive look, will I be able to obtain that look with 17 inch wheels and 235 45 17 tires without lowering the car? Or do I have to lower it so the maximum amount of gap will be covered?

 

I have 19" wheels with 235/35 tires on the car in my sig. Judge for yourself if that's the look you are after and go from there

edit..

for some reason my sig picture isn't showing up...my avatar is also my car, not sure if there is a way to view a bigger pic of it

 

I've seen Z's with bigger tires on them to fill up the wheel well gap. I wouldn't recommend it, t just makes the car look funny. But if you really want to do it, 235/45 R17 isn't a whole lot bigger than the stock tire outer diameter (25.3" vs ~25").