If i were you, i would wait for the MacBook. I don't know the I/O on the eMac, and I cant be sure everything runs smoothly under leopard.
Is it just me or does it seem like a lot of people on this website install leopard on really old computers? I mean I am on a 1.25 G4 just like the eMac, but people seem to be installing it places it shouldn't be. I just look forward to the more stable snow leopard.
There should be two thinks that look like headphone jacks on your emac. One has a picture of headphones next to it. Use the other one. Go into system preferences>sound>input. Choose line in. Plug in your DS and have it make sound, see if the computer "hears" it by looking at the bars. Then use garageband to record. Good luck!