Ubuntu 12.04 recommends using a minimum of 1GB of RAM memory; but with 3GB of RAM it is working very fast and is more preferred, and Ubuntu 12.04 may still be used on many computers for maybe a decade, or until there are no longer any updates available for it? When; Ubuntu is used or duel booted with another Ubuntu OS, on a hard drive it seems to work very good.
The minimum requirements for Ubuntu 12.04 is 384MB of Ram, a 700MHz CPU processor, and 8GB of hard drive area.
Installing and configuring Ubuntu for the first time. ( New Users Guide )
XP to Ubuntu with an 8yr old Hacktop - Computerphile
Ubuntu Tutorial for Beginners
The Linux File System - Explained