you can go online to www.tirerack.com and choose your model car, they will show you what rims they have that are available for your car. They will even show you what your car would look like with the rims. If 20" show up as available then you can put them on your car and have no issues of rubbing at the same time you can maintain your current ride hight. Remember that 20" rims will make the car ride a lot rougher... a lot. If you think there is a difference between stock and your 18" wait till you get to 20". another web site to try is www.discounttiredirect.com... also great places to order your tires for a fraction of the cost then you would find at the local tire store.
I couldn't own a car like that, there are too many speed bumps and road irregularities where I live. So easy to crack rims and damage suspension components. I personally think those rims are 20's though.
Maz, I think you are right, but the fact that they are deep dish rims, the tires are really low profile, it being lowered, and the picture being shot from the angle it was shot at, makes the wheels look huge.
I think your car looks pretty sick just the way it is.