What to do with my garage roof?

[OzNimbus]

Ok folks, quick question:

What do I do with my roof? I was planning on sealing the underside with a layer of drywall. There's 2*6 rafters to attach to.
The question I have, is it really necessary to insulate the cavities?

I'm planning on having separate cielings (insulated) for my internal rooms.

Here's a pic

 

[John Sayers]

I'd rum battens at 90 degrees to the trusses and drywall that if you are going to put a new ceiling below. But you must put insulation above the drywall to stop the cavity above resonating.

cheers
john