Just as a mildly contrary note, I had massive issues with a 2gb Sandisk card. Not sure exactly what class it is, but it's certainly not SDHC or anything fancily fast. (This card actually came with a HTC Hero nearly three years ago so I'm sure it's genuine)
Primary issues were:
* Random hangs. Basically stuck on the last page you turned, with the end of book icon in the top right corner. Pressing the Home button dropped back to the main menu, but the SD card contents then vanished until a hard poweroff was performed.
* Loosing content. Sometimes decided to loose all the content on the SD card until a hard reboot was performed.
Tried this on several firmwares from 1.9.10 to the ill-fated 2.0.0 release, and never saw much improvement. When it worked it was fine, but I never knew when it was going to drop out.
I'd be interested in comparing our good/ problem SD card lists to that of the Raspberry Pi, as I suspect there's probably a controller issue somewhere as opposed to a size issue.
I'm (almost) tempted to try a custom kernel update with the Kobo GPL sources we have available to see if updated low-level drivers do anything for us
-Leezer-