Is there a difference between a secure digital card and an MMC?
Difference between Secure Digital & MMC?
Will they work in n-gage?
Yes, there is a difference! SD is thicker and has an extra contact.
SD does not work in the Ngage!
SD cards are "newer" than MMC -- a hair bigger, but can generally hold more data (ie... 1 GB SD cards currently exists, 2 GB coming out soon), are more "secure", faster data transfer speeds between device and card, and a few other things.... check out: