I have an iMac from 2007.

Its early a machine that isn’t ever really pushed but with recent OS upgrades and only running 3GB ram its starting to show its age.

I have a ram upgrade in the post and will see how it is running 4GB but I’m contemplating taking my OS back to 10.6 or even 10.5.

Bad idea?

I have an external hard drive that I back up to so, if I understand things correctly, I should be able to restore music, pictures etc.

If I understand things correctly I should be able to reboot(?) from one of the install discs for either 10.5 or 10.6 and then install my files from my hard drive via the time Machine back up facility and my hard drive.

Have I gotten that right?

What is it running at the moment? If it isn’t El Capitan, and you have the option of installing that (should be able to with a 2007 iMac) I’d try that first. It is supposed to be more efficient & most people have had good results even on old hardware.

Maybe. I’m not sure an older OSX will restore from a far newer update.

I’m not sure I’d want to run such an old OS as 10.6 without the various security patches etc. since then.

@ zilog6128 – Interesting. Will try the upgrade as its free etc and see how things fare.

@ IA – I understood it- perhaps incorrectly that the older OS’s were still supported from a security standpoint.

I believe support for Mountain Lion is still possible but Lion has dropped to EOL and is no longer supported. Mountain Lion will be going that way soon.
Leopard and Snow Leopard will have big security holes now but age may make those not worth exploiting for the crackers. The Safari browser will not work on a lot of websites, it will be Firefox or nothing as Chrome has dropped support for older OSX installs.
Assuming you won’t be banking or shopping on it. You’ll be ok to look at STW but stay away from Torrents and Porn as any payload in a download may well own your machine.