I dont think your choice should depend on your CPU speed. If anything matters, it must be adding a graphics card to your system. Unfortunately, you dont have much to choose from. I would almost say OS3.9 should be just as fast as OS3.1 when you select a graphics mode that suits your machine. And that also means you dont need fancy icons and solid window dragging if you dont have a graphics card. if you turn all that off, OS3.9 is for sure the better OS. A tweaked OS3.9 really is a great 'little' OS, something I even prefer over OS4.
