SD cards are (officially) limited to 2GB.
SDHC's run up to 32GB.
Current card pricing has quite a few Class 10's running as cheap as the same brand Class 4's or 6'es so it probably won't make much sense to go for a slow card.
BTW, I vaguely remember there might be a way to upgrade without a flash card...
(Check the older threads.)
Still, a flash card really adds a lot of capability to the PB360. Just remember that nature abhors an empty flash card.