If you have an RB30e, go a 2.25 or 2.5 inch exhaust. Retains back pressure, and sounds nice.
3" systems are mainly for turbo cars, or high powered N/A cars.
Definatly see Mark at Exhaust Tech, the 2.5 JUN BL cat back on mine is great, and has good customer service too. I had a problem with the exhaust bracket falling off, buzzed down there,and was out of there within 10 minutes.