In cases such as this, you'd probably want the "stated value" insurance as others had suggested -- you'll have to justify value with a copy of the window sticker as Jared suggested, and you'll have to decide just how much you're willing to pay. Some companies will do "stated value" only on collector cars driven only x-number of miles or less (not a primary vehicle), so you'll probably have to shop around unless someone pipes-in with some of their experiences.
Team Saleen will provide a photocopy or scan of your window sticker for free, and at least that's a starting point for value.
As Greg said, NADA does not list the Extreme, but the information on the SC may help: http://www.nadaguides.com/
and click on the "Used Car Values" link in the right column.
It would be better to over-insure than under-insure... at least until better values are published, but basically you'll looking at window sticker minus x-amount of depreciation.