I think that the M8688LL/A model you have is the single 867MHz. You can confirm this by looking at the sticker on the back of the computer below the graphics card.
Airport comes in the original 802.11b which will not do you any good because of the lack of speed and that it can only connect with passwords using WEP (not suggested) or WPA with a firmware update I believe, but not WPA2.
The FireWire 800 model had processors with speeds of 1.0GHZ, dual 1.25GHz, and dual 1.42GHz. This is the only model that accepts Airport Extreme with 802.11G. I believe this will connect using WPA2 security.
If your MDD G4 is NOT the FireWire 800 model, I would suggest using a PCI wifi card. I believe something like this would work:
[Edited by Moderator]