Your wired network is capable of at most 100Mbps. Don't count on more than about 60% of that for any length of time. The hub may be giving you a bit of a bottleneck if your G4 is connected to that. But here's the REAL problem: even if you have one of the new, 802.11N AirPort Extreme bases, wireless networking takes a lot of overhead (channel management, error correction, etc.), which eats into your actual throughput, giving you less, usually MUCH less, usable throughput. If you want to back up ANYTHING through your network, it should be on a wired network.
Now let's burst your bubble. The theoretical maximum throughput of USB 2.0 is 480Mbps, or more than four times the theoretical max for 10/100 Ethernet. And even though you will get less than 480 averaged over any lengthy activity, it will still be much faster than Ethernet. Firewire has a max throughput of 400Mbps, and supposedly has a better streaming capability than USB, so it would be better for backups too.
Sorry, but you're either going to have to manually connect your PowerBook to a wired network and wait a while, or connect it to a USB or Firewire drive and wait less. Or leave things the way they are and wait and wait and wait...