I would suggest you stick with 17" wheels. A few of the people on this board have tried 18's and are not happy with them. The SR20DE is not a powerhouse in stock form. The bigger and heavier your wheels are, the slower you're going to go. Most of the people in this forum are running 17x7's with 205/40's or 215/40's. Take a look at the chart on g20.net and you can get an idea of how the different wheel/tire sizes will affect your gearing and speedometer error. Also remember that the lower profile you get, the less rubber there is to cushion the rim against potholes.