you can do the ghetto cold air intake by changing your stock air box a zip tying the filter in place. Check out b15sentra.net for instructions. Its in the how to section under performance mods its good for a few ponies and alot of noise.
get a K&N filter from autozone (45 bucks) tell them its for a 2001 SE. relocate the knock sensor i left a post telling how to do it. find a buddy who does exhaust work. And this one is old school but it works get a smaller diameter drier hose (black) mount it in the lower grill remove the intake snorkle from the airbox and run the new hose to the airbox you might have to cut the box open a little to make it fit better. this will act like a cai and ram air (sorta)