Q: Have 10.8.2 What is latest OS I can get?

Depends on the year of your iMac. But the latest OS out is El Capitan 10.11.1.

Here are the requirements

OS X El Capitan - Technical Specifications

If you have Mountain Lion, you can install El Capitan.  Whether you want to or not, is a personal decision.  Lost of RAM and an SSD can improve performance.

The 2 places I’ve seen recommended most to buy reliable RAM are below. I have purchased RAM several times 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 now have 6GB installed on a early 2008 iMac supposedly limited to 4 GB and noticed an improvement.

Crucial

Other World Computing

Check to make sure your applications are compatible.

Application Compatibility

Applications Compatibility (2)

El Capitan 10.11 Compatibility information

Do a backup before installing, preferable 2 backups on 2 different drives.

I have used Crucial for multiple Macs and have never had a problem. Their site can "scan" your computer and tell you which RAM will work for you too! They are cheap and pretty fast as well.