It depends on what your goal is. Do you want to see a history, or just periodically see what your usage is? I use GkellM System Monitor. It does not keep a history for the RAM; it only shows your current usage, but it also displays CPU and other data.
You could also type the command "free -m" into a terminal to see detailed information on the RAM.
Conky is my system monitor of choice. It's light-weight, but it also takes a fair amount of manual configuration. If you Google around you should be able to find other people's configuration files on the internet, and you can use those and tweak them as a base. There should also be some packages for system monitoring applets for your Desktop Environment if you want closer integration. You might even already have them.
Well, you might want to try out htop. It's an interactive process viewer which gives you more detailed information about specific processes rather than your whole system. It's a console-based utility but has a nice interface uses Ncurses regardless.
You can try loading up Gnome applets and Gnome desklets. There are some sytem monitors in the packages. The one which I am using has bubbling water graphics to illustrate the ram usage. I am also using another one which uses burning fire graphics to illustrate cpu usage.
