similar template but with different image in header (twentyten)

Hi guys,

I want to split off part of my blog (different topics) and want to keep using 2010 theme but with a different header image and possibly a different menu, blogroll, side widgets etc.

I know that I should start another blog but I don’t want a whole bunch than one to ‘maintain’ :-). I want to run the main site as a portal almost.

Does anyone know how to make a duplicate of the template (with the above mods) and then save it so that I can chose it from the ‘list’ (default template, modified template one, etc) when I write a post or page?
Thanks in anticipation….
Cheers
Ian
p.s. It’s the twentytens themes fault, the new menu builder is awesome!