IMPORTANT: Most systems in this category will be 32-bit only, outside of lucky owners (Ex: P4 HT 630 2MB). However, machines with a Core 2 Duo or Quad do explicitly support 64-bit. If you attempt to boot a 64-bit system on a 32-bit only system, the system WILL NOT BOOT. If you intend to use 64-bit SW, verify the processor supports it!

There are fewer distros available that work well on "legacy" systems (Ex: Core 2 series and older). You will want to use a lightweight distro such as Lubutu for these systems. Check the minimum requirements for these systems to ensure you will not run into issues.

RAM limitation note: If you have a system that cannot support 4-8GB, a 32-bit distro is (generally) better for performance. However, this will limit application support. Using a 32-bit distro on a 64-bit capable system should only be done in the event of performance issues due to a hard RAM ceiling due to chipset or BIOS RAM limits.

For these systems, your best bet is a distro like Lubuntu or Xubuntu. Other lightweight distros are also suitable, but verify they will work with your system.