[jcrx]

You want the JDM 98+ ITR header, the others are normal headers, nothing special. And as far as the difference in catbacks, that would be diameter and bends, a lot of crappy manufacturers use crimp bends, instead of mandrel, and they imped flow, and the design of the muffler is pretty important, not to mention, factors like do you want it loud or quiet, so you need to look into the resonator size. Cheap catbacks also will run into fitment issues. OF course if you get the right header, the collector is going to be bigger, so you're going to need a cat that can bolt on to it.