If you don't need access to the pre-2001 Mac OS 9 applications, you can buy the retail version of 10.5 as explained in this tip https://discussions.apple.com/docs/DOC-2275
and apply the combo 10.5.8 update after installing 10.5 (the link is on the tip).
Otherwise you'll want the retail release of 10.4:
https://discussions.apple.com/docs/DOC-2541
And apply the 10.4.11 PowerPC combo update after getting the retail 10.4.
Either of those will give the 17" Powerbook the most bang for its buck. Make sure you install the proper RAM of at least 512MB on the machine if you do upgrade as my FAQ below explains*:
http://www.macmaps.com/badram.html
Note: you can't install the operating system via USB. You'll need to get the actual optical disc media.
A Firewire DVD burner from http://www.macsales.com/ can also install it if the optical drive is failing internally.
Not all Firewire DVD burners are built the same.
USB can't boot Mac OS X on PowerPC Macs such as the Powerbook.
* Links to my pages may give me compensation