Extreme SDHC Class 10 vs. Extreme III SDHC Class 6 at 30MB 200x

Hello,
just bought a nice camera, with HD capabilities. the seller warned me, to get a fast SD so it will not become a bottleneck for the video bit streaming.
so… i read a bit  and tried to understand which standard or marking will help me choose.
* According to WIKIs, the class determine some kind of minimum trasfer rate (6 MB for class 6 etc).
* According to Trancend web site the class 10 cards is up to 20 MB/sec
* SanDisk  cards - there are so many cards it’s very confuzing:
   1) extreme 3 which are marked class 6
   2) extreme 3 with 30 MB/sec also marked as class 6
   3) newer extreme also 30MB/sec but marked as class 10

someone can enlight me??