Start with a y-pipe. It's the best $ to hp increase you'll get; somewhere in the neighborhood of 8 additional hp. Next go for the intake and exhaust. I've had both a mid-pipe intake (places the filter behind the battery), and currently an Injen intake. Both are vast improvements over stock. You should also get good gains from getting a new cat-back exhaust; the VQ likes to breath. The underdrive pulley is also a nice addition. You could even swap a 2000-2001 intake.
But with all those mods, I still don't anticipate a 50hp gain. The main problem is that the automatic transmission is the week link when it comes to adding power. You can however have one built, or modify the valve body and add a trans cooler.
The way to go if you're looking to make a Maxima/I30 fast, is to get/swap a manual trans. This a link with instructions on how to do the swap.
http://www.vqpower.com/v2/modu...id=30 With a manual, and most of the mods mentioned above, you could add just a 35 shot of N2O and have a very quick car.