Sometimes it's the brand that matters
There are really poor quality SD cards out there. For example: I have this really cheap Transcend 2GB SD card that I never got an ISO to boot from.
There are some known brands that produce good quality SD cards like Hama, SanDisk etc. There is no need to buy the ultra-fast ones though, since the EXI-port of the GC is technically limited to 3,2MB/s anyway (that it never reaches). The only reason to buy one of these is the increased speed to transfer stuff onto the card from your computer, but seeing that they cost twice the money... your decision!