Unity users can easily monitor CPU and Memory usage by using indicator-sysmonitor application.
To install it, you just need to add alexeftimie PPA repository using these simple command:
sudo add-apt-repository ppa:alexeftimie/ppa
sudo apt-get update
sudo apt-get install indicator-sysmonitor
Note that this doesn't work on previous Ubuntu 10.xx releases.
Alternatively, you can install Conky by running the command:
sudo apt-get install conky