There are many indicators for Ubuntu which display the CPU, memory or network usage in a graph in the Panel. If you find these visualizations distracting, then you might like the simplicity of Indicator Sysmonitor. It just displays the CPU and memory usage percentages as text in the Panel.
To install this indicator, add the PPA repository ppa:alexeftimie/ppa and install from the repository using the package name indicator-sysmonitor. To start the indicator from the Dash, type System monitor indicator.
Tried with: Indicator Sysmonitor 0.4.1 and Ubuntu 12.04.2 LTS