Reducing RAM Usage in Ubuntu
Like it or not, Ubuntu has grown to use more RAM than it did in past releases. When Canonical began using Unity and took away the Unity 2D option in Ubuntu 12.10, many users with lower-end machines started to notice. However, configuring Ubuntu just the way you want and like it is still possible. That is part of the spirit of GNU/Linux, and it's very easy to make Ubuntu use much less RAM and feel quicker on any computer.
Use a different desktop environment Unity is large part of the RAM usage when you boot up a default Ubuntu 13.04 install. And if you use KDE, you might need even more RAM (they recommend 1 GB for Kubuntu). To avoid this, you can easily install a different, lighter-weight desktop environment. I use openbox, which is very light on RAM usage. There are also many others that use much less RAM than Unity, including fluxbox, JWM, i3, awesome, xmonad. The last three are "tiling" window managers, which will get you into another intriguing aspect of desktop environments.
Use non-default applications Ubuntu ships with some pretty heavy applications for doing every-day tasks. Take Thunderbird, for example. Thunderbird is Ubuntu's application for email. It uses quite a bit of RAM and CPU. It uses 59 MB of RAM without even setting an account up, while bouncing between 20% and 80% CPU usage. I usually use my email provider's web interface, but when I want an offline version of my email I use getmail to back it up as an mbox file and mutt to open it. Mutt is a command line email client. When I use mutt to load my 2013 email (808 messages), it only uses 1.3 MB of RAM.
Ubuntu also uses a resource-hungry file manager called Nautilus. I like to replace this with PCManFM. It uses a little over half the RAM that Nautilus uses -- 4.1 MB vs 7.4 MB.
Still too much RAM usage? If Ubuntu still comes out too heavy in RAM usage after the changes mentioned above, try checking out a different GNU/Linux distro. Debian, on which Ubuntu is based, uses less RAM out of the box. Here's a Crunchbang thread where a user has Debian using only 23 MB of RAM with a full desktop: Crunchbang SUB 100 club And Arch Linux with openbox uses even less than Debian. So if you're really into reducing your RAM usage, check those out.