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The X standard used by the flash memory industry is the same as that used by the compact disc industry:
One "X" = 150KB/sec
Usually fast cards are only a real help if you are frequently transferring large quantities of data over a USB connection or if you are using a camera that benefits from high write speeds.
The power drain is negligible for any SD or CF card (flash memory only, not microdrive CF cards etc.).
Aside:
As for Sandisk's Ultra II card claim...186,000 miles/second is the speed of light. I'm not sure electrons can travel quite that fast yet in conventional electronics.
I tested out a 512MB Sandisk Ultra II SD card in a USB2 card reader recently, and it averaged only 7MB/sec or so. One of my 1GB normal speed Lexar SD cards wrote at 6MB/sec in a test a day or two earlier.
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