Don't upgrade unless you need to or want to.

One of my older computers was getting bog down with viruses and too many applications so I decided to wipe it clean and start anew. So I tested the Windows 8 preview. I liked the interface and speed but there were a couple of problems, mainly lack of drivers for many devices. For instance, there were no drivers to support the Intel Graphic Chipset which meant that videos ran very slowly because it was not using acceleration. When the preview expired I wanted to upgrade to the release version of Windows 8 but the lack of drivers was even more severe: there was no support for my wireless card which meant no Internet connection.

Ultimately, I reinstalled Windows XP and it works beautifully. I didn't even try Windows 7 because I have it in another computer and I like it well enough but it requires much more memory and a better CPU to work smoothly.

To summarize:
- Windows 8 in an old computer may run into compatibility issues.
- Windows 7 requires more memory and CPU.
- Windows XP is very robust, mature OS that will serve you well for many years to come.

If you want to try them by all means go ahead. If you don't need to, then waiting is better, after all there will be new and better Operating Systems in the future by the time you need to change.