for cards 8 GB or above, i don't think you can get anything slower than a Class 4.
here's one way to look at it:
16 GB SDHC card, Class 4:
15 Mb/s = 1.875 MB/s, so it would take about 140 minutes to fill the card.
16 GB SDHC card, Class 6:
20 Mb/s = 2.5 MB/s, now it would take around 105 minutes.
it would be interesting to know what the speed of the standard card that comes with the 2Box is...