Have Magic Key 3, what else do I need?

Fresh from a wonderful experience with homebrew on the PSP, I decided to explore the possibilities of my DS. Unfortunately, even after googling many tutorials, I still feel somewhat lost. I decided to come here, because I know that this site brought new life to my PSP, and was very informative for assisting me in providing this second breath.

I have the following:

• DS System (Firmware v.1)
• Magic Key 3 (256M + 64M Zip)
• 1 GB Sandisk SD Card
• SD Card Writer
• Official NDS Game

I was wondering what I would still need, and the cost of such items. I've come to some conclusion I need a GBA flashcart, but was wondering what the least expensive route is. Any help is greatly appreciated, as right now the MK3 is serving as a piece of black plastic. With a shiny sticker. I'd hate to have the money spent on it go to waste, but if there really is a far better route, I'm willing to abandon the current path.

I've come to think I need one of these, but was wondering if I was correct, as a lack of research initially led me to believe I only needed the MK3. (I realize my stupidity) $50 is not too much to pay to make the Magic Key do something other than sit around, but I was wondering if it works. And if so, are any capabilities reduced? I was hoping to play GBA homebrew as well, so was wondering if I the Magic Key and Neo in combination could play NDS and GBA homebrew. If so, any limitations?

Thanks in advance again to any knowledgeable people who help me out of this conundrum.