There seems to be so much confusion going on about SD cards, and I thought it would be worthwhile to properly explain the use of SD-cards.
The microSD is a format for removable flash memory cards which are commonly used in cellular phones, but also in handheld GPS devices, portable media players, digital audio players, expandable USB flash memory drives, and for Nintendo DS flashcards, along with digital cameras.
It is the smallest memory card available commercially; at 15 mm × 11 mm × 1 mm (about the size of a fingernail). A point of confusion can come from the difference in the microSD and the newer microSDHC format. The SD and SDHC share a similar form factor, but not all devices are compatible with the newer high capacity format.
When you shop for a new microSD-card, you need to be aware, that those cards come in different speed-ratings. A closer look on the packaging will reveal: Class 2 / 4 / 6 as well as 133x / 66x.
The class defines the minimum write speed for a completely empty SD card. That means if you were writing to a new SD card that has never had any data on it how fast would it write.
Class 2 = 2 megabytes/second
Class 4 = 4 megabytes/second
Class 6 = 6 megabytes/second
So in short Class 6 is what you should be buying for fast write speeds. On top of this there is also the speed rating of the card itself. This is based on how fast the microSD card can READ the data off the card. You will see this usually described as 32x, or 66x, or 133x, or something with an x and the end.
These measurements are based on the original CD-ROM drives which could write 150kb/second. So 32x means 32 multiplied by 150kb/second or 4.8MB/second. The following table (sourced from Wikipedia) provides you the detail:
I strongly recommend the Transcend, 16GB, class 6 card which I have been using for a few weeks in my HTC (G1) Dream.