Easy answer is - "depends on what profile your running". Having said that however, the size of your rims should make no difference on how much you can lower it if you're running with the standard total rolling radius. I agree with what the other guys have said - you need 225/40/18's. This gives the same rolling radius as the standard setup (normally 195/65/15's or 225/45/17s - like I've got).
I dropped mine 35mm, using Eibachs and although its a bit harsh over bumps, I get a good ride most of the time, and no rubbing (in fact the glove compartment sometimes springs open when I go over big potholes - I need to get the catch strengthened or something!).
Hope this helps.