If the Coolpix support 256 MBytes SD cards, It will also support cards of 512 Mbytes.
The reason is that the cards are formatted using FAT16 or FAT32 file system.
FAT16 support 'disks' up to 2 Gigabytes and FAT32 up to 2 Terabytes (they are not available yet

)
The high end DSLR use FAT32 to format their memory cards.
I think the coolpix will format using FAT16. It means that you have to take care if you want to buy a card of more than 2 Gigabytes. Then a firmware update will be necessary.
More info:
http://www.ntfs.com/ntfs_vs_fat.htmRegards,
I forgot to tell that you should ALWAYS format the card using your camera, NEVER format using Windows. Format regulary to get more space.