Two Prong outlet replacement

I live in New York City, and have come to realize that I should replace
some 
old 2 prong outlets. I am sure that I only have the hot and neutral in
these 
outlets, supplyed through grounded metal flex.

Is it allowed to use the the armor as a ground or do I have to find 
replacement 2 prong outlets  or GFIC outlets?

I know that NYC wrights it own rules, but I am also curease about NEC
rules, 
so please note which you referance.


Thank you in advance

Stephen