I would like to use the normal key in my 2008 Sentra, but when I do, the car alarm goes off (I'm tired of replacing the smart key, when my wife loses it!)

You can have the Smart car system taken off.. The dealer or a GOOD mechanic can do this..Then you can get regular keys made much cheaper..

Thanks for the answer, although I wonder if the cost of taking off the smart system would more or less defeat the purpose of using the conventional key (ie, to save money). Getting and programming a new smart key is about $250.

The I-key system cannot be changed but the alarm should not go off if you're using the mechanical key.