I think it can only take in a max of 640, one slot with 128 built in, and the other with a max of 512.
To boot from USB, plug a drive in that has a working Mac installation that is bootable on your machine. Then press the power button, right after you hear the startup noise, press the Option key. In the boot menu that appears, select your USB drive. Note: It will be very slow because you're using USB 1.1.
that is the normal procedure but, it still can't boot from USB
Beginning with the Power Mac G4 (AGP Graphics) and the iMac (Slot-Loading), two new features to USB are most apparent: support for USB audio devices and booting from USB drives.
PPC Macs can indeed boot from USB. I've done it many times. Even Apple says it can be done: https://support.apple.com/kb/TA25908
Note the second paragraph, quoted below.
But were not talking about a PowerMac G4 AGP are we. Were talking about an iBook G3. Again like the HD comment context is everything
"Beginning with the PowerMac G4 AGP And iMac (slot-loading)"
In 1999 both the G4 AGP's & slot load iMac G3's were introduced.
The G3 iBooks (assuming he's on about a dual USB model) were introduced in 2001.
Read the quoted text again, but this time don't skip the word "beginning". The slot loading G3 iMac came out in October of 1999. This means that any Mac made after it's introduction can boot of of USB. For first 500mhz G3 iBook can out in May of 2001. Last I checked, 2001 comes after 1999. Following simple number logic, one can see that the OP's iBook can boot from a USB drive. Again like the HD thread, you wording and lack of through reading is key.
The HD thread was about SSD not ports, you made the leap to ports.