I recently purchased the new Pontiac G8 GT, and have been using every spare minute looking for replacement wheels. The Australian Pontiacs use the BMW bolt pattern but a different hub size, so not every BMW wheel will work on the Pontiac. So far the most popular wheels for the car have been the Breyton Race GTS and Race CS wheels in either 19" or 20" fitments in black or silver finishes.
Some of the Breyton wheels are available in two widths to allow for a wider tire on the rear axle, but the wider wheels have been known to be so close to the fender lip that they may rub over large bumps. I'd recommend giving us a call to discuss the options and clearances the larger wheels would require.
When fall arrives, we will also have several options for snow tire and wheels for the car starting in 17" wheel and tire sizes.