Setting up render layers is a great idea for flexibility, but it likely won't help your RAM problem.
Yes it will because you will be rendering less at once and thus the memory footprint will be less.
As far as what to leave on I would do a beauty and shadow for each layer and then spec if you need to tweak it. That's probably all you will need. There is a free tute here at SM about render layers
Light linking is where you tell maya to only use certain lights to light certain objects. The default is to try and light everything but light linking will make mean that only certain lights will be used to light certain pieces of geometry.
Alan
Technical Director - Framestore
Currently working on: Your Highness
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