Other brands and capacities of cards may work, but Nikon cannot guarantee their operation. Check with the manufacturer of the third-party card for compatibility information. Nikon recommends keeping one of the approved cards available for troubleshooting. ------
But they make no distinction between SD Extreme and SDHC cards.
There is an ** next to the Sandisk 4GB approved memory card on Nikon's spec sheet which says that the 4GB must be SDHC compliant. From looking at Sandisk's web site, the only memory card that is SDHC compliant is called the SDHC card... other cards are not listed as such.