My basic question is: What's the highest I can go with MacOS, without having to install the Sims 2 Super Collection?

Background info: I've come to the conclusion that my current Mac OS (10.8 Mountain Lion, if you can believe it!) is no longer sustainable in today's internet world. Until recently I was able to open emails on my Gmail account; that is no longer true. So... I need an OS that will allow me to access modern websites, but also support my current Photoshop version (which I absolutely cannot afford to upgrade). Seemingly I can notch up a few Macintosh levels and still have CS4 work, but how will that affect my Sims game? I'd really rather it be a seamless transition, where I wouldn't have to jump through a thousand hoops to keep all my CC working, but I realize this may not be possible. My current Sims 2 game was brought over from an even older laptop, via an external HD, so I didn't need to re-install anything, just make sure all the parts got to where they needed to be. Anyhow, that's my story.