^^just like i mentioned earlier; i hereby quote the GoPro HD manual
"USING SD CARDS
The HD HERO camera is compatible with 2GB, 4GB, 8GB, 16GB, and 32GB capacity SD and SDHC cards.
While all speed cards work in the camera, GoPro recommends minimum Class 4 speed cards for reliability
when shooting HD video. GoPro’s own testing has shown improved reliability in high vibration activities with
name brand SD and SDHC cards."
and from my own and others experience, some brands just don't work...
brands that almost definately work are kingston cards (GoPro sells these)
and sandisk cards...
class 4 is recommended and i find 8gig to be more than sufficient; get 16gig if you're planning on shooting more than 1 battery with a card or if you wanna play on the safe side....