has reputation for running well on older harder.Linux has reputation for running well on older harder.
This is true, but it's not pixie dust that you can sprinkle on any old piece of hardware and expect the Prince of Good Performance to appear.
Generally, expect Linux to perform about like Windows does on the same hardware. Or more accurately, expect to perform like Windows does before it's bogged down with an investation of spyware and adware.
Mark recommends the following as minimum specs for a Linux desktop experience that will fill comfortable to use for everyday applications including web browsing, e-mail, office applications, watching video clips and basic music and photo processing.
The absolute minimums are usually much lower. Check the details for your preferred distribution of Linux before you install.
If you have particularly old hardware you'd like to keep alive, be prepared to use it a little differently.
Damn Small Linux is an easy to use LiveCD full of slimmed-down applications. It gives you the option to add larger applications like OpenOffice if your computer can handle it.