I learned a few things about the differences between these two cards and how little is known about them at the Cingular office near my work. I went in to ask them what kind of a card I needed and was told that the Nokia 6620 takes an SD card.

SD cards, or Secure Digital Memory Cards, are the same size in every dimension as MMC, or Multi-Media Cards, except that the SD Cards are slightly thicker (2.1mm vs. 1.4mm for the MMC cards). The MMC card has only 7 contacts and the SD Card features 9 contacts. The SD Card also has physical indents on its sides and a slide lock for memory card write protection.

It turns out that MMC-compatible devices, like my Nokia 6620 phone, aren't compatible with SD Cards but the reverse is true that SD Card-compatible devices can use MMC cards. SD Cards also 'feature', if you can call it that, a little-used DRM file protection scheme that the MMC cards don't have.

Both formats are available up to 2GB but SD cards are much more prevalent in consumer digital cameras and portable devices, such as the Treo and as such, the MMC format may be a bit more expensive.

I hope that this information helps someone avoid purchasing the wrong memory card for their phone or protable device like I did. Check out your phone or camera's manual before heading off to Office Depot to purchase a new card for your device.