Fry's Electronics has the DLink DWL-G122 Revision B with Firmware v2.02 for $49 and then you get a $15 mail in rebate so the DLink only ends up costing $34.

Those who buy a Dual Core/Quad Core G5 and didn't custom order their Mac wire Airport should get the DLink instead. Even though you can see the Airport slot in the Dual Core system there are no rails to support it and the Antenna wire doesn't fit the Airport Extreme card. I'm really blown away that Apple made the Airport Extreme a customization option only (like the Bluetooth) for the Dual/Quad Core G5 systems. You can order just the Dual/Quad core specific Airport Kit from SmallDog.com for $99 but they warn you a technician should install it. Buying the DLink is a much better, cheaper, and safer alternative.

Good luck and I hope this is helpful information for everyone who bought a Dual/Quad system thinking you could add the Airport later!