Serious answer: You haven't provided enough information to receive an a meaningful answer. If you keep the wheel width, offset, tire width, and overall tire diameter within the same range as the stock wheel & tire specifications, you can increase the wheel size without impacting fender & suspension clearances.
What are the width, offset, and tire size of the 20s you are considering?
ressurecting an old thread for a new question : what are the specs on RX8 wheels? i know the bolt pattern is 5x114.3, but what is the offset and width?
edit: ok i found what i was originally looking for (18" x 8", +50 offset). now i'm curious, does anyone know how much these weigh?