Preface: My first experience with Mac's is with 10.4.8. So, if you are using an older OS, any directions I give related to settings may be incorrect.
From what I found - the video card in your PowerBook is the ATI 9700. It was a decent card in it's day. Actually still not bad, just not up to date. But, at that time there just was not the call for widescreen resolutions as part of the default settings. It should be able to handle them, just may not have it as one of the default options.
Am assuming that is the Apple 23" cinema display you currently have. Am guessing the PB does not have a default option of 1920 x 1200 since there were few 16:9 displays and as far as I am aware there were no 16:10 aspect displays in production at the time. Am guessing all of the default resolution settings on the 9700 are probably 4:3 aspect ratio. This is why you're getting the stretching effect. You will probably have to play around with different resolutions to find the one that's the best with the PB. There is a 3rd party software (it was mentioned and linked on another post here not too long ago, just couldn't find it with a quick search) that will allow you to manually set the proper resolution.
With the Samsung, and most all 720p HDTV's, it should accept incoming signals of at least these three: 720 x 480, 1280 x 720 & 1920 x 1080. It would then scale these to it's actual 1366 x 768.
When you have the PB connected to the cinema display, do have any of these widescreen resolution options available, or are they all 4:3 aspect ratios? If they are all 4:3 options, we'll probably need to find that 3rd party software.
edit: found it - original link by Aptmunich -
DisplayConfigX
and here is the original
thread - we never got a report back as to whether it worked or not