If there's no OS on the new internal drive at all, then no keyboard shortcut is necessary, though pressing and holding the option key at startup will give you a GUI listing of all bootable partitions connected to the computer.

If there is nothing on the new disk, all you have to do is connect the external drive to the USB port, boot the iBook, and it will search until it finds the drive connected, if the USB drive is supported, which, iirc, is dependent upon the controller chipset.

Also, I hope you used Carbon Copy Cloner to copy the DVD (and make the drive bootable), or it won't work at all.
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