From a lot of the posts on the Pocketbook forum here, there are many problems with Sandisk cards for some reason. I have a Kingston 4GB micro SDHC card and I haven't had a problem with it.
From my understanding SDHC cards are available in bigger sizes. I doubt I'll ever use all 4GB of mine, so keep that in mind before you go and get a really huge card. Also, some cards are faster than others, the higher the "class" number the faster they are.