PAE kernel and 4GB of memory

[Howard Wilkinson]

I am looking for a definitive answer to the question of where the PAE 
kernels become useful. I have seen various articles that mention needing 
PAE kernels if you have more then 4GB of physical memory in a 32-bit 
processor environment. I have also seen statements that say you need 
them if you have 4GB or more of memory. Now which is right? Also, even 
if you need a PAE kernel because the last few bytes are not addressable 
when you have exactly 4GB is this useful or is the trade off of larger 
page tables and pages going to eat any benefit of being able to address 
these few bytes and if so when does the PAE kernel become useful?

Howard.