Ok, now that you have your CRF450X, you start wanting to modify it. This is perfectly normal, and now you have to confince yourself, or your wife to spend the money on your toy. This may be a battle, but nonetheless a battle worth fighting, After all, what would your bike be with stock exhaust? Quiet? Slow? Of course!
One exhaust with a lot of good reviews is the FMF Powercore 4 Exhaust, in either aluminum or titanium. Either way, it will best the stock system in weight. Be good, and make sure you get the spark arrestor version to keep USFS people happy.
Adding this exhaust sounds easy, well because it is. Anyone can put the exhaust on, but the trick is to also open up the airbox and rejet the carb. This bike is already severely choked up, and adding the exhaust will just lean it out even more! A JD jet kit and some slight airbox modifications via holes will net you more power, crisper throttle response, and that nasty off-idle bog and popping on deceleration will be eliminated.
Of course, it will be a bit louder and mileage will go down but that is only because you are twisting the throttle more, and further. I know when I modify, I do the same, but worth the while!
See you on the trails!