Personally, I would rather have a separate button to start the operation. It's very easy to accidentally pick the wrong option from a menu ring, and there could be other options the user might need to select before pressing that 'go' button. Of course I can't speak for your application, but having the extra button really hasn't ever made a difference to me in any programs I've used or written.
As far as getting it to work the way you want, some people have used an invisible listbox below a disabled ring. Use a mouse event on the ring to make the listbox visible. Then when you click an item from the listbox, you can use a mouse up event to determine what element was selected before making it invisible again. This has been discussed on the forums, there may be some examples if you search around.
Message Edited by Marc A on 10-03-2007 08:51 AM