HF100 / HF10 uses SDHC Class 4 or higher
You need an SDHC card or cards, Class 4 or higher. Capacity is up to you. 8GB seems to work out at around about the best value for money. Transcend, Patriot, Kingston, A-Data all make good SDHC cards that are less expensive than Sandisk's offerings. There are good deals to be had if you shop around. And make sure you work out some kind of backup / archival strategy for your clips (a search on this forum should bring up plenty of information on that).
Bottom line: so long as the card says "SDHC" and "Class 4" or "Class 6" then you're good to go.
Andrew.