Oh boy...welcome to the world of DS hacking...
First off, there's currently about a gazillion and a half different flashcards out there, each claiming to be the "newer and better" version of the R4. Most of them are actually crappy knockoffs that fail after several weeks of use. My best recommendation is to stick with the R4 or and M3 Simply (they're made by the same company) and be sure that there's no weird suffix attached to the name. If you want to look into it further there's actually a good comparison of the legit cards on Wikipedia. http://en.wikipedia.org/wiki/Nintendo_DS_storage_devices