You might be able to upgrade to El Capitan, although you will need to install Snow Leopard first.
Check that your computer is compatible with Mountain Lion/Mavericks/Yosemite/El Capitan.
To check the model number hold down the option/alt key, go to the Apple menu and select System Information.
MacBook (Late 2008 Aluminum, or Early 2009 or newer) model number 5,1 or higher
MacBook Pro (Mid/Late 2007 or newer) model number 3,1 or higher
Your Mac needs:
OS X v10.6.8 or OS X Lion already installed
2 GB or more of memory (More is better - 4 GB minimum seems to be the consensus)
8 GB or more of available space
Check to make sure your applications are compatible. PowerPC applications are no longer supported after 10.6.
Application Compatibility
Applications Compatibility (2)
El Capitan 10.11 Compatibility information
Also check to make sure there is a compatible driver for your printer.
Do a backup before installing, preferable 2 backups on 2 different drives.
Snow Leopard Purchase
Use the combo update to save time in Software Update.
10.6.8 Combo Updater
El Capitan
The 2 places I’ve seen recommended most to buy reliable RAM are below. I have purchased RAM several times and a hard drive from Other World Computing and have always been very satisfied with the product and service. They have on-line instructions on how to replace the RAM. OWC has also tested RAM above what Apple states is the maximum. I had 6GB installed on an early 2008 iMac supposedly limited to 4 GB and noticed an improvement.
Crucial
Other World Computing