Best bang for your buck is a y-pipe from either budget(
LSS Exhaust - Burlington, Ontario) or warpspeed.
either of those will give you about 15hp at the wheels. Next would be intake. I ONLY recommend JWT pop charger or the stillen pop charger. Why you ask, because the rest of them are overpriced and dont do much (CAI kill top end). Dont get an injen they suck. Ive tried almost every intake setup on the maxima and the best is JWT/stillen pop charger with the velocity stack or stock with a K&N filter.
After that you can choose to do catback but they dont give very much gains. I recommend 2.5" or smaller.
You may notice your power drops hard after 5500rpm that is your intake manifold. You can upgrade to either the MEVI(From foreign market maximas) or the 00vi from the 00-01 maxima vq30de-k. The 00vi is better but the MEVI rocks too. Either of those are best utilized with an extended rev limiter (JWT ECU).
I know a thing or two about modding maximas. I had every single bolt on, on my 98 maxima 5spd, fully bolted naturally aspirated just wasnt enough for me so im going for boost. Garrett T61 + 10psi boost!!