You could try to restore the Powerbook using the Time Machine backup. Don't know if that will work, probably depends on the Mac OSX version you were running on the intel Macbook (10.6 does not run on PowerPC). Insert the installation DVD for 10.5 and after a while (when it asks you if you want to move files from another Mac) you're able to select "Restore from a Time Machine / Capsule".
If that doesn't work out you can just install the machine and attach your USB drive. The backup on the disk should contain a file called ".sparsebundle". If you double-click it it will be mounted as a drive on your machine. Navigate to it, select the folder with the date of the backup and drag over the files / directories you want restored. If you restore 'Library' or other system folders don't forget to logout/login or reboot.
