I have 3 SD cards for my Pentax *ist DS digital SLR, and they're all "normal" 1GB cards from different manufacturers that I found on sale in the past 11 months or so. I've taken at least 5000+ pictures of everything from butterflies hovering at The Butterfly Farm, in St. Martin on vacation, lots of portrait modes, to full-on action shots of NASCAR cars racing in the California 500 race at Fontana with those cards, and I can't say I've noticed that I need to step up and get a high-speed SD card.
Supposedly, if one is taking a lot of multiple-frame-per second photos, it makes a difference, but I haven't pushed that envelope yet with this camera, and I've had the ASA hung out to 3200 speed for the action shots of the NASCAR cars, guys, and gals. I'm sure some of the professional photogs here will chime in about this topic, but I don't see any need to spend 3X as much for a high speed SD card as long as your camera has the ability to shoot what you're trying to accomplish without one.
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What's the difference between reality and fiction? Fiction has to make some sense.