Sure, but in case it matters you’ll probably be throwing good money after
bad. There is very little I can think of that CS6 can do better than CS5
and CS6 is now pretty out of date for any modern digital output formats
such as EPUB.
I'm almost forced to upgrade to remain compatible with my client. My concern is if my Mac and OS meet the requirements to upgrade to CS6.
They do, but what happens when the client(s) move to CC? Of course doing
so opens a new can of worms for you since 10.6.8 is unsupported for
anything later than CS6.
You should start budgeting for some upgrades sooner rather than later.
BTW, that machine should be able to run Yosemite fairly well. I have it
running on a 2008 Macbook with a Core2Duo and 4 GB of RAM though I don’t
use it for production.
If you go that route, do a clean install.
So you recommend upgrading the OS to 7 before upgrading InDesign to CS6.
I recommend doing it at some point, yes, but that’s your call.
CS6 is fully supported on 10.6.8.