I have an M3 Perfect, and it has built in functionality for playing Game Boy roms through Goomba. Basically, all you do is put the goomba.gba file in the root directory of the SD card, and when you choose a .gb file from the normal interface, it'll automatically run it through the emulator. It works great with old black & white games, but I couldn't get it to do Color games for some reason.

As for "big" games, the M3 Perfect (if I remember correctly) has 32MB of room for loading roms, which was as large as GBA games ever got. Some of the older flash cards might not have had this much, but as long as you get one of the later models, I imagine you'd be fine.

Finding specific GBA flash cards isn't so easy these days though. No one really manufactures them anymore, so aside from old stock, your only option might be eBay.

--Zero