If its the original DVI model (667-800MHz) then i would probably suggest 10.4 since its has a slower processor and an older Radeon 7500 than the last TiBooks. Personally, I've chosen to go with 10.4 on both my 667 and 867MHz Tibooks. Its plenty fast and since there is still support for the newest versions of iTunes, Safari, and many other popular applications, I didn't see any pressing need to force 10.5 on them.