i have apowermac G3 (with firewire). can i add any airport card?
-->Literal answer: Yes. Since only Apple makes the AirPort card you can use any one. So when you go to the Apple Store and they have 20 AirPort cards on the shelf you can use any of them because they're all identical. AirPort Extreme is a different card though, and you cannot use that.
what about 3rd part stuff.
-->That's a different sotry. All those PCI wireless cards will will fit, but drivers can be an issue. Shop carefully. You'd probably just be better off buying the Apple card.
Also , what is the advantage to an airport base station vs other (cheaper , d-link etc.) ones?
-->Typical Mac integration, easy to set up, wireless printing, Looks. That's it. Honestly the AE base station isn't the finest piece of hardware ever made.