Some Power Supply Questions - Multiple Rails or Single Rail w/ multiple Outputs?
Forget the 7812. If you're going to use a switcher to create a higher voltage first and then use the 7812 to go to 12V, you might as well use the switcher to go to 12V directly.
If the output voltage is close to the input voltage (sometimes below, sometimes above), you want a buck-boost regulator, like National's LM5118. The Webench Designer creates a design to your parameters.