They say the minimum requirements is 2chip + 6meg fast (8meg total ram), but I've got it to run successfully on 4meg fast. I haven't tried it without fast ram. It would probably run, you would probably just lose a lot of features.
OS 3.9 system requirements
Os 3.9 is just as fast as OS 3.1. There's a lot of little tweaks here and there to the OS, but nothing has been dramatically changed.