A 400Mhz G4 wouldn't have an Airport Extreme card, it would have to be a Sawtooth cause the Yikes! never had an Airport slot. Also, OSX will run if you bump the RAM to 512 (it will work with less but is a waste of time) and halo will not run on any mac under 1.2GHz.
Also the processor the sawtooth uses doesnt work with a Beige G3 or an iMac, only with other sawtooths (sawthii?) and maybe a few later G4s. Now, they also made the Yikes! G4 which used a G4 ZIF that -was- compatible with the Beige G3, and the B&W G3, however this machine wouldn't have an Airport slot so there is a bit of confliction.
Also the processor the sawtooth uses doesnt work with a Beige G3 or an iMac, only with other sawtooths (sawthii?) and maybe a few later G4s. Now, they also made the Yikes! G4 which used a G4 ZIF that -was- compatible with the Beige G3, and the B&W G3, however this machine wouldn't have an Airport slot so there is a bit of confliction.