How do I dump a GBA game?

You can use a flash linker like flash2advance ultra if you have one :P

 

I've read that you can do with if you have a Wifi Connection, DS, and a DS flashcart like R4 DS. Don't remember for sure though because I didn't try it.

 

You don't even need a Wifi connection. I have used a homebrew application in the past that can simply dump the GBA cart on the Micro SD card inserted in your R4/M3 Simply/whatever. I can't remember what it was called, though...