All my cards had at least some amount of FAT32 partitioning on them. My current setup is:
7450mb FAT32
500mb ext4
50mb linux swap
The diff between classes 4/6 is
minor but stated in the spec. Are you running a 4 now for sure? Most cards are 2. The jump between 2-4 or 2-6 is significant but the jump 4-6 is less so. If I had 4 I might have stuck with it, except I read that many/most 6 cards have wear-leveling logic built in. If true, this will extend life on my journaling/swapping setup. I have read that anything beyond class 6 would be a waste on g1 because the bottlenecks would occur in hardware somewhere.
People doing these things would get their money's worth from spending extra on class 6:
* lots of video recording, particularly HD
* swap or journaling fs on the card
* move big files to/from the card frequently (syncing to iTunes or whatever)
Other people could probably be happy with a cheaper, 2x larger card for the same money.