I need to be sure that if I purchase a newer, more powerful, Mac in the future that I can uninstall the software from the one and install it on the other.
Is that possible?
Sure. You simply deactivate it on one machine and then install and activate it on the other. Of course nobody at this point can know about technical issues that may occur with future versions of Creative Suite vs. then current versions of OSX.
Can we purchase it once and install it on multiple computers in our household?
You can install on as many machines as you like until your ears start bleeding, but you can only activate on two at any given time. if you need more, buy more licenses. See the EULA. And due to the "oldness" of your Mac, make sure it is actually able to run CS6 by checking the tech specs.