Well, the extension is not everything we value a domain. right?
The name on the left part of the domain should be considered at the same time.
For instance,
I've got mp3shop.com and mp3.us. It's hard to say which one is worth more. Because mp3 is short one word name, if this website is just for americans or american music that's perfect to choose mp3.us.
However, other people like mp3shop most because it is a .com and will easily attract vistors throughout the world.
btw, .org does not rank 3rd in my opinion. there are lot of more worth something like .info .co.uk. .biz
go to sedo.com or afternic.com or sale section in this forum. you will graduallly get the sense of a domain's value.
good luck.