Changing header.php based on user levels

I would like to stop causal viewing of my blog, and make people login to be able to read/see anything.

I would also like to hide (or change the content of) the header.php bar at the top of my theme based on the user level.
Can I get some pointers, examples please?

Ive been trolling through for a while and havn’t seen anything pointing me in the right place.

Thanks
Malcolm H