Well, I ran a couple benchmark tests on the SD cards I had lying around, and I think Class 2 will be ok. I used HD Tune to test these cards.
The OCZ 2GB 150x I had been using capped out at 19.1MB/s.
A Sandisk 4GB SDHC Class 2 card capped out at 10.4 MB/s.
The SanDisk 2GB Class 2 I bought capped out at 10.4 MB/s.
A SanDisk 2GB without Class 2 written on it was also 10.4 MB/s.
The Canon 32MB card that came with my camera capped out at 9.8 MB/s.
An old SanDisk 128MB card I had capped out at 1.7 MB/s.
Well, at least the whole class 2 thing is consistent. I think it should be sufficient for the Wii.