If I buy this
Super Card I can just pop a 2gig SD card (loaded with GBA games) in it, put it into my GBAsp or my brother's DSlite and we'll be playing all those roms, no questions asked?
Basically. I'm not sure about the Supercard, but the M3 requires the roms be patched for things like save states and whatnot, but there's really not much to it and the patching software is included when you buy the device.
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Is there some firmware flashing involved? What's this passkey stuff?
This is for playing DS games through a Slot 2 (GBA) device. Since the DS is only supposed to be able to play DS games from Slot 1, you need to either flash the firmware to get it to read from Slot 2, or get a device that will sit in Slot 1, boot up, and tell it to read from Slot 2.
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I'll be able to do the same with DS games, provided I have the ROMs?
It works about the same as for GBA, aside from the aforementioned passme thing.
Honestly, if you're mainly after DS, I'd pick up an R4 (slot 1, microSD) instead of or in addition to the Supercard. It can be used as a passme if needed, and there's no patching needed for DS games.