for the ds. you can get a few different types of cartridges. Ive heard of the M3 and the R4. i think there are more though. How they work is that you put a micro sd card in the cartridge (looks like a ds game cartridge) and you put that in your ds. After that, everything works like a regular game. U just select the game and play

. I have one for my regular DS (not ds lite) and it works fine. U can get them pretty cheap nowadays and sometimes u can get them bundled with a memory card. I got mine of dealextreme.com and it was perfect. Good luck. They are really worth it.
Edit* also you dont need to mod your ds in any way. all u need is that cartridge.