Standard Airport Extreme will work with PowerMac G5, but:

1. There are obviously two versions of this card. One (A1026) is not supported by MorphOS, while the other (A1027) works OK.
2. The solid aluminium case of PowerMac G5 doesn't really work well with WiFi, so you need an additional external antenna to have some range of signal. Now... Apple in its own unique way didn't care to put a standard SMA connector, but used a proprietary antenna that's pricey as hell. There's a solution though. Instead of buying a genuine one, use some generic antenna with MCX male connector (or an adapter from MCX male to SMA).