Just ordered a Canon HF100 and need a SDHC card(s) for it. They recommend class 4 or faster, but I'm wondering if class 4 will be fast enough? Sandisk class 4's show a record speed of 20 mb/s and the camera records at about 17 mb/s (from what I read). So would the class 4 be enough? Or should I spend the extra and go with class 6?