X pipe for Mustang?

I have a 99 GT. I have recently installed a SLP Loudmouth Catback and was wondering what x pipe would compliment my new cat back? Catted/non catted and where is the cheapest place to find one. I usually buy at www.americanmuscle.com but the pricing seems a little high for the X pipe. I was also wondering if x pipes will affect smog testing when it comes time to renew my CA plates.