18" or 17" and how low can you go...

2000 3.0L EX Coupe:

I want to drop the car and add 18" to them. A friend said that he thought 18" would rub underneath. I am curious to know if this is true. Also, the car has a smooth ride now, how low can I go without jeopardizing the ride too much, since I will have 17" or 18" the ride will be less comfortable already, correct?

Basically, I want the car to look smooth but drive smooth also...I don't plan on racing it or doing anything crazy with my baby...

Thanks for the input...