1. Yes, it is a good idea
2. You can use the 2nd (empty) disk drive to install Ubuntu on it, just make sure to select the 2nd drive when you are installing to don't wipe out your windows installation. Ubuntu should automatically detect your Windows installation and it should configure it as dual boot via Grub
3. It depends
Maybe Ubuntu is not the best choice if you are interested in performance on relatively old hardware
https://help.ubuntu.com/community/Installation/SystemRequirements#Lightweight_GUI_alternative_.28Xubuntu_and_Lubuntu.29