For a light Ubuntu distro either Xubuntu or Lubuntu would be a good choice but honestly with the specs you mention you should be able to dual boot and run pretty much any Ubuntu distribution you want.
You'll likely need to resize your existing partition with gparted
to get the 80GB of free space you mention as unallocated prior to installation.
muru makes a good point in that you could simply add a light desktop environment to your existing installation and not dual boot at all but simply choose the environment you want from the login screen. This is of course a matter of preference.
Xubuntu
from terminalsudo apt-get install xubuntu-desktop
– Mudit Kapil Mar 12 '15 at 14:49