Passscard is the perfect flash card boot-up solution for DS LITE & NDS . It is compatible with most types of flash cards including M3, Neo, SC & G6 etc.
I've just received a Passcard 3 in the mail. Nintendo DS homebrew, here I come! I'm particularly excited about NitroTracker and ScummVM. Though I've only got a GBA Movie Player and a 128MB CF flash card from a camera, that seems enough to run most of this stuff so far. Unfortunately, it just sticks out from the Nintendo DS Lite by an inch or so - almost half the depth of the handheld itself! I think one of my next purchases may be a more compact and capable SuperCard MicroSD, which fits flush in the Nintendo DS Lite and accepts MicroSD expansion cards.