I'd recommend that you think long and hard about putting 20's on your car, however. I have the 18" HRE 540's, and I'm extremely happy with them. 20" wheels aren't going to contribute to performance, and they will (imho) interfere with the handling and stability aspects of your car. Remember, the suspension is designed with a certain sized wheel in mind. I'm not sure that a 20" wheel would look cool enough to risk it. If you want to get rid of the extra wheel well around, say, an 18" wheel, then get some modified springs and drop the car 3/4 of an inch. I've seen this done w/ 18" HRE's, and it looks pretty slick.
Just my .02,
Andy