Upgrade internally? No, it isn’t. The next laptops to be released following the DLSD PowerBooks, the first-gen MacBook Pros, used a PCIe bus for AirPort Extreme 802.11n. The PowerBooks used a PCI bus.
If you want to upgrade a DLSD PowerBook to run off 802.11n, there are two paths: use the PCMCIA CardBus to add an 802.11n card, or buy a 802.11n USB adapter.