There are different types of SD cards: SD/SDSC, SDHC, SDXC, SDUC. Your camera is compatible with SD/SDSC and SDHC cards that are 4GB or smaller. See the Nikon D80 user manual for details (page 122).
A firmware updatefirmware update may add support for larger cards, as well as fix other issues. However, all SDHC cards are 32GB or smaller, so your 64GB card would likely still not work.