You do not, necessarily, need to run Linux.
That OP is making confusing and conflicting recommendations.
If you are running older, expensive software under OS X 10.5.8 Leopard, it will continue to run under OS X 10.6.8 Snow Leopard, due to OS X 10.6.8 Snow Leopard's use of a special IBM PowerPC emulation software that was part of OS X 10.6, but is no longer part of later OS X versions.
These are avaialble without having to install Linux on your Mac.
Depending on the type of apps you are currently using under OS X 10.5.8, for later versions of OS X, their ARE both free, open source options and cheaper pay options for many older, pricey apps that no longer work on OS X 10.7 Lion and later OS X versions.
There are free, open source Office style apps for later versions of OS X.
Alternate image editors that CAN replace Photoshop
Drawing/painting apps that CAN replace older versions of Illustrator, Corel Painter, AutoDesk AutoCAD.
Open source apps that can process RAW image files and replace Adobe Lightroom for many features.
So, if you decide to upgrade your OS X versions to the pay version OS X 10.8.5 Mountain Lion, there ate many options for different older third party software apps.
Another thing you will need to do is the versions of Apple Safari that runs under OS X 10.8.5 is no longer supported and is unsecure.
The ONLY up to date and secure web browser for OS X 10.8.5 Mountain Lion is the Waterfox web browser.
For all of your other older software, there are cheap, cheaper of free near equivalents to be found in Apple's Mac App Store for OS X.
Good Luck to You!