The main thing to remember here is that you do not alter the overall tire/wheel diameter. In other words if the stock wheels and tires are a total of 25 inches tall... then ANY wheel and tire combination you pick should come out to 25 inches tall too. This will keep your speedometer accurate as well as the ABS/ESP systems working well.

I would not go smaller than 18" wheels if you have an SRT6 as anything smaller may hit the calipers. I'm not sure how small you can go on the base and limited models.