Which OS?

Apple no longer sells Mac OS X 10.5.x, so you will have to go through a third party. Several sellers on amazon have it, for quite a bit more than the original purchase price.

EBay looks like the right answer for getting OS X 10.5.x, especially if you want to continue to use your applications and documents.

If your machine was a PC, it would be a candidate for a lighter-weight Linux distribution, like Xubunu or Lubuntu. These are Ubuntu variants that use a desktop with lower resource requirements. You can also install Ubuntu and a variety of desktops (LXDE, XFCE, etc.) and try out their performance.

The PPC CPU in your iMac isn't supported by the official Ubuntu release, but is "community supported." You can get releases from the Ubuntu site here: https://wiki.ubuntu....owerPCDownloads

If you want to use your machine for Web browsing, you might find a Linux distribution a better option. You will get more choices of up-to-date browsers.

There problems with any choice, however. I don't know of a Java 1.5 runtime for PPC, for example. You will have to make a choice based on how you want to use the machine.