SDHC is a card family and has nothing to do with the size of the card
From wiki.
"The SDSC (standard-capacity) card family, commonly known as SD, has an official maximum capacity of 2 GB, though some are available up to 4 GB.The SDHC (high-capacity) card family have a capacity of 4 GB to 32 GB. SDXC (extended-capacity) card family have a capacity starting above 32 GB with a maximum capacity of 2 TB."
Just buy a microSDHC card. 8GB class 4 will be enough for your phone. With class 6 u will get marginally better performance on a xcd28,wont be noticible. If u want to be future proof get a 8GB class 6 else go for class 4.
Look for a circle with number 4 or 6(depending on which class it is) on the card. Sandisk,kingston,transcend are good brands to look for.