Assuming this PowerBook is a 15- or 17-inch variety, not all may be lost, fortunately. There is a dual-band
PC Card/PCMCIA solution which does work natively on PowerBook models running Leopard (and possibly earlier, though I haven’t tested those).
Provided you have one of those models, this 802.11n card is treated by OS X as a native AirPort card (in fact, in this setup your PowerBook will recognize
two AirPort cards installed). Because it’s dual-band, it does work in the 5 GHz range, though I find real-world transfers range between 150–225Mbps at their best — slightly lower than the rated 300Mbps speed on the box. On a PowerBook, this will be an appreciable speed bump over the 54MBps of the OEM AirPort Extreme internal card.
If you’re running a 12-inch model, though, your only wireless path would be to find an 802.11n USB dongle rated to operate at 5GHz (avoid all 802.11ac USB dongles, as non of these drivers were written for PowerPC usability). Unfortunately, I’m unable to advise on where to pick a surefire 5GHz USB wifi solution in 2021 which works with PowerPCs. They are out there, but I’m not up to pace with what’s still being made.