Lighter in CPU/Memory Usage: Lubuntu or Xubuntu
Your case studies are indeed challenging.
Take your Grandpa PC
Pentium 2 and 64Mb RAM.
Lubuntu minimum requirements are 128Mb if using the alternate installer - 256Mb for the Graphical installer.
The recommended RAM is 384Mb just to run the LiveCD.
Therefore - I'm afraid, you will not be able to install a graphical environment on Grandpa PC. At best you might get a text only "server" type install via a minimal ISO - i.e. you need a minimum of 64Mb to run the minimal CD.
The "old meat":
That is more promising:
Pentium 3 with 128Mb RAM - you should be able to squeeze on Lubuntu via its alternate CD.
However the minimum requirement for Xubuntu is 256Mb with 512Mb as a recommended RAM size.
I've experimented with various "low" RAM installs. I've never tried 128Mb - but in general, I install from a minimal ISO CD and install various components manually - for example, SLIM, LXDE with apt-get install --no-install-recommends to ensure no excess packages are installed.
Typical packages you should look out for if you go this route are described in the linked question below.
Stick with a lightweight browser such as midori - abiword should be ok for wordprocessing.
Flash and general movies will struggle with your video card. However playing music should be fine (see below)
Links:
- https://help.ubuntu.com/community/Installation/MinimalCD
- How do I install LXDE / Lubuntu?
- Lightweight music player
In my experience there are a few things to consider.
First, and most important, do you (or your users) like the interface ? It makes no sense to "force" xfce or lxde on them. So, ask for their input on this.
Second, for the most part, the window managers all use about the same in terms for RAM and CPU. The caveat here is you need to compare apples to apples, if you enable 3d / special effects you will take a performance hit.
Most of your resources are taken by services or applications running in the background. To save on resources, disable as many as possible (do you use Seahore, VNC, etc). run gnome-session-properties and disable as much as possible.
Data: On my netbook ...
Gentoo + Openbox - When I first log into openbox (not LXDE), it uses 70 Mb RAM.
Ubuntu 12.04 - Unity 2d - Uses 300 mb RAM
With that said, firefox runs at about the same speed on both window managers.
So are all the bells and whistles of Ubuntu Desktop worth 200 Mb ram ?
Answer to that = personal choice.
Test it for yourself in Virtualbox or KVM, give your guest 512 Mb RAM, boot the live CD.
I would install bare console Debian, then with apt-get will install incrementally some window manager like OpenBox (it will install X as dependency)
Then will check how it is, if i need taskbar icons etc...