At the moment biggest available SDHC cards are 16GB. (At least I haven't seen 32GB available for purchase yet?) 16GB size has been reported to work with the OS2008 or latest OS2007.

Largest standard SD cards I have seen are 4GB, while the SD standard itself support only up to 2GB. So to play safe you should not exceed 2GB if buying regular SD card. Anything above 2GB should be SDHC to guarantee compatibility.