Q: what is the upgrade for MAC OS X 10.5 and how/where do I get it?

You need a Mac OS X 10.6 DVD to run that software or upgrade to any newer version.

(74410)

The first step in Upgrading... is to Snow Leopard = OS X 10.6.x

It is Not available as a download... It is a Paid Upgrade.

Do this first...

Check that your Mac meets the System Requirements for Snow Leopard...

Snow Leopard Tech Specs

http://support.apple.com/kb/SP575

If so... Purchase a Snow Leopard Install Disc...

http://store.apple.com/us/product/MC573Z/A/mac-os-x-106-snow-leopard

Other countries...

http://support.apple.com/kb/HE57

After the Successful Install, run Software Update to get the latest updates for Snow Leopard.

Be sure to make a Backup of your Current System Before Upgrading...

Next... Check that your Mac can run Mountain Lion...

Upgrade to Mountain Lion

With Regard to ML...  Check here for compatibility of 3rd party Software you may be using...

http://roaringapps.com/apps:table

You should also consider more RAM... Get the Maximum you can for your Mac...

It is important to get the Correct and Matching RAM

See Here  >  OWC RAM  >  http://www.macsales.com