1, do NOT use MG to play GBA games. PocketGBA is *far* better. Please read
my roundup of GBA emulators if you want to know why.
2, in the latest version of PocketGBA, you can play ROM's up to 32 Mbytes. If you really have that much memory (that is, your PDA is not a WM5 one like the x51v), you won't have problems playing big cartridges.
3, you can't convert flash ROM into dynamic RAM.