intake: between the Stillen intake and the PRCAI, it's almost the same. it seems like the SI gives you a slight edge in top end vs. the PRCAI.
on the other hand, PRCAI will give u much better intake sound because of its metal intake plenum vs. stock rubber one. i believe there was an article regarding this in MD.com, so go there to find out more about this.
y-pipe: you have 3 choices for your car. you have Stillen, Cattman and Warpspeed. Stillen y-pipe is a ceramic coated pipe and Cattman and Warpspeed use T304 stainless-steel. if you live in an area where it snows, then you should opt for either the Cattman or the Warpspeed y-pipe. if not, Stillen y-pipe is fine. as far as the prices go, Warpspeed is the cheapest, then Stillen and Cattman is the most expensive. noises from y-pipe vary, but my Stillen y-pipe was very quiet when used with stock exhaust. i did not experience the 'bees-in-a-can' noise people complained about.
exhaust: you have many choices for your car. you can either go with a Stillen muffler(rear-section) or a HKS muffler(again, just a rear-section) and combine with either one of them with a Stillen B-pipe to make ur own cat-back. Stillen muffler is infamous for its loudness though. You can also opt for the GReddy cat-back, which requires very little effort to bolt onto your car(these things fit like a glove). what's the next choice? you can go to local muffler shop and custom make everything(do this if you are strapped with cash). but remember one thing though, if you live in Califonia(i don't know if the same applies to other states), any modification to your exhaust system is now illegal. it doesn't matter if you buy GReddy or some Mike's muffler, it is not legal anymore. so do this at your own risk.