I have a very low end PC, 512MB, and I'm thinking of switching to ubuntu. But I want the lightest one possible, I did a few searches and came across Lubuntu. Are softwares installable on ubuntu also installable on Lubuntu?
Lubuntu and Ubuntu use the same software repositories.
Note though that some programs require GNOME runtime libraries, or KDE runtime libraries, and those runtime libraries will also take up space on your drive, as will the program. These programs may also not work 100% with your hardware, but there's no way to truly tell.
One way to find out what other packages would be installed with a package is to simulate its installation first:
Both distributions use the same repositories. So, software that can be installed on ubuntu can also be installed on Lubuntu. At most some extra gnome packages will be installed (this will mostly be taken care of by synaptic or software centre). But with low ram, Lubuntu would give a better peformance. Also be aware of the fact that Lubuntu 12.04 is not an LTS as opposed to ubuntu.