Thanks Francesco for posting these nice images. I'm pleased to see that my efforts to maintain Sweet Home 3D for old systems are not useless. Not a huge effort, but still, at each release or when I change some special stuff in 3D, I spend a few minutes on an old MacBook G4 to test if it works.
What Java version do you use (see About dialog box)?

Keeping on maintaining the old PPC code in future

I don't plan to remove support in the close future, but I can't guaranty anything.

Enable GPU accell for rendering

This has nothing to do with PowerPC support, and if GPU accleration for photo rendering ever happens, it will be probably thanks to some third party libraries which might not exist for PowerPC, and the fact that such libraries support or not PowerPC won't be the number one criterion of choice.

Add to the Mac PPC build the YafaRay renderer to vastly improve renderer speed and quality

This would imply to have the XCode environment to compile libYafaRay and libYafarayJNI for PowerPC with no guaranty that it's feasible. I don't have such an environment.

Possibility to deactivate the "simultaneus 3D rendered view" to speed up performances on older systems.

The 3D view is not activated when a SH3D file hiding the 3D view is opened, i.e. if you want to deactivate the 3D view in an opened file, you should hide it by changing its size, save the file, close it and reopen it.
In the next version, updates will be deactivated automatically in the 3D view when it's not visible, either by changing its size to zero or by displaying back a zero sized 3D view in the main window. The second behavior will be quite handy because Sweet Home 3D keeps in memory the sizes of the 3D view when displayed in the main window and when displayed in a separate window. Therefore, if the size of the 3D view displayed in the main window is zero and the size of the 3D view displayed in a separate window is not zero, using the cmd + alt + Y shortcut (Ctrl + Alt + Y under other systems) will display/hide and activate/deactivate the 3D view.

I found myself keep on chaging "Wall Thickness" and "Wall Height" parameters in the Preference panel which is quite annoying and time consuming. Couldn't it be worth to allow that in a more easier way?

In case you didn't notice, you can change the size of different walls at the same time by selecting them then choosing Plan > Modify walls menu item.

Maybe directly from the top menu (some integer textfield next to the create wall button)?

This would take a lot of room in the toolbar and as you want two text fields, this would take even more room because it would also require some labels to distinguish them. By the way, I don't plan to add a second line in the tool bar.

A button to temporarily stop the 3D view rendering would be a first step to at least solve the problem when working with large projects

I won't add such a button and hope that the solution I propose above will be enough for you.