miniSD is the physical size of the card, SDHC is the storage format. The N810 uses miniSD-sized cards and can use both SD and SDHC storage formats.

The confusion comes because SD is both a size and a storage format.

If you buy a good brand of memory card, it normally comes with its own adapter which should work fine (some of them even come with several adapters for all the different sizes).

One thing though, try NOT to buy memory cards on ebay as there are a lot of counterfeit card sellers on there. Counterfeit cards can stop working quite quickly, I know some people who were caught out by this and they were very annoyed.