I'm getting ready to replace my exhaust manifold with a header off of ebay *the mani. has HAD it*...while doing this I'm also thinking about picking up a catback to go with it. My question is this...if my catback's diameter is larger than the stock dimensions-will I need to replace the cat also??
the catback's diameter is going to be either 2.5 or 2.75

why go larger in the back at all?

from what i've read it's best to grab a catback off a turbo model. It's cheaper, it still increases power, and it looks stock so you don't got to worry bout the cops as much.

You might have to gind a little bit on the exhaust flange to get the oem cat flanges to fit right but nothing too bad. I would also recomend going with a GST 3" exhaust now if you plan on going turbo later. No point in buying 2 exhaust. If you are staying na I say go with a 2.5".

thanks for the input guys. i will probably end up going with the 3" catback-to make things easier in the long run.