Well the first thing you should invest in is any maintainence that you may need like sparkplugs, changing old fluids or anything else that may need to be done. After all of that you could move on to performance.
1. Aftermarket ypipe- Theres plenty of information on the forum about this so just search around a little. Basically it adds 20 whp to our cars.
2. Intake- Either a WAI(warm air intake) or CAI(cold air intake) will do. The best is a place racing style CAI. Look for them on ebay. I'm ran just a cone filter on the stock intake for years and still made good power.
3. B-pipe-This is the pipe after your cat. convertor. This increases hp by 12+-.
4 00VI-There is also alot of info about this around the org mainly the all motor section. It is the intake manifold from a 2000-2001 maxima. It adds 20 peak hp and 60hp at redline. They can be found in the for sale section for around $350-$400. I've sold about 6 full kits already.
There are many more mods out there but these are the most worth while performance mods in my opinion.
