Welcome! It looks like you have been around for a couple months!
You have a couple options for lowering your Altima and improving the suspension. If you are on a budget, you can do lowering springs. I would recommend Eibach or Tein as those are the most popular around here. If you are looking to go all out you can take a look into a coilover setup from BC Racing or Tein. Take a look through the forums and our parts directory for help in making a choice.
As far as warranty voids go. The Magnuson-Moss Act states that a dealer must honor a warranty if the part in question is not in direct relation to the warranty claim.
Take a look and read more here...
http://www.free-lemon-law-guide.com/the ... ss-act.php
Have fun
