My understanding is that the "CLASS [246]" stuff is simply the speed rating of the device. It means [2] MB/s, [4] MB/s and so on, Minimum Write speed. The SDHC (and done right) is the important part. I've heard that there are a lot of SD cards floating around that are re-branded counterfeits. To be safe, you might want to buy from an established dealer.

For example, I recently purchased an 8GB mini-SDHC from NewEgg:

www.newegg.com/Product Product.aspx?Item=N82E16820220295

For $32 USD and it works great in my N810