macos - What is required to make RAM modules fully compatible with each other?

Two questions:

1. 
   

   Can one memory module be better than the other, if both are at the same speed (667 MHz) and both are DDR2?
   I'm asking this because I found two modules with identical specifications, but one cost $40 while the other costs $160.

   
   


2. 
   

   Is all RAM compatible with Mac?
   My iMac accepts, DDR2 SDRAM at 667 MHz. Does this mean that ALL desktop DDR2 SDRAM at 667 MHz would work with my iMac? Or do Macs specifically have some kind of additional requirement? (i.e. "Mac compatibility")