Also from the Kobo website Kobo Touch Specifications:
Memory Expansion Up to 30,000 eBooks with a 32 GB SD Memory Card
Since you can not have an SD (SDSC) card with > 4GB this would have to be a 32GB SDHC card.
NOTE: according to Wikipedia some of the 2 and 4GB SDSC cards were created out of specification and may not work with all devices that support SD (SDSC).
SDSC = SD Standard Capacity
SDHC = SD High Capacity