This is entirely dependent on what you're using your computer for.
For most users, the Facebook addicts and the email checkers, more RAM isn't the solution, a new computer is if you've got an old one and are constantly suffering from poor performance. For gamers of any sort or people with heavy workloads, more RAM is always beneficial and 8GB is the least I would suggest. Most games are cutting above the 4GB RAM average that most computers have, especially MMOs.