Minimum Requirements for Ubuntu Desktop edition

Processor/CPU       300 MHz (x86)
Memory/RAM          256MB
Hard drive/Storage  4000MB/4GB
Video Card          Capable of VGA 640x480

Recommended Requirements for Ubuntu Desktop edition

Processor/CPU       700 MHz (x86)
Memory/RAM          384MB
Hard drive/Storage  8000MB/8GB
Video Card          Capable of VGA 1024x768

Server requirements

Processor/CPU       300 MHz (x86)
Memory/RAM          64MB
Hard drive/Storage  500MB
Video Card          Capable of VGA 640x480

http://en.wikipedia.org/wiki/Ubuntu