Check out club lexus. They have a sticky on the sc430 forum about wheels with a link to an awesome write up on all the different wheel and tire combos including sizes, offsets ect.
I too live in Chicago (Westmont) and just purchased wheels and tires. I went with 19 inch based partly on the cost of tires. The 19's can be had for ~$170 each at tire rack, but the 20's are $280 or more.
Here's a pic I just took at the dealer with my phone right after they were installed. If you look in the window, you can see my old pie plates. Mine are 255/35/19 +45 offset in the front and 275/30/19 in the rear with 45 offset. The fronts rub, just barely, with the front upper control arm. I had to take them off. I ordered a 5mm space from H and R and it should be here tomorrow.