pretty much marketing hype. just make sure you buy at least a class 4 card
I just ordered a Canon Vixia fh200 camcorder. I haven't owned a camcorder in 20 years, so a lot of this is new territory (although there's been plenty of internet detail to read up on and get me to this point).
My question is, can I buy an SDHC card (I'm looking at 8Gb or 16Gb) or should I pony up the extra $$ for an HD SDHC which sells itself as better for HD recording. Is there merit in this, or is it just marketing hype?

Chowhound
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GameFAQs
GameSpot
Giant Bomb
TechRepublic