The Ubuntu system requirements are well documented at https://help.ubuntu.com/community/In...emRequirements
Ubuntu Netbook Edition
If you have a netbook and a supported graphics card, using Ubuntu Netbook Edition (formerly "Ubuntu Netbook Remix") is an interesting option. It has a nice interface adapted for smaller screens, and a selection of applications aimed at netbook users.
Ubuntu Netbook Remix is designed to run well on netbooks with typically minimal specs, i.e.:
* Intel Atom processor @ 1.6 GHz
* 512MB of system memory (RAM)
* 4GB of disk space
* Screen of 1024x600 resolution
* Graphics chipset with support for visual effects
If your hardware does not meet the minimum specs, then I recommend a smaller distro (like Puppy Linux).