A GPU probably won't help and you've got all the RAM most of us ever dream about. The slower, older CPU might be an issue but you haven't told us anything about your projects. If you're doing 4k, you could be have a processing problem on any Mac but most of us old timers have successfully been doing DV, 720, and 1080 on everything from a Mac mini and iMacs to ye olde PowerBooks and we've been doing it for a long time.
Lots of stuff can cause a freeze. I'm guessing you've got some CMYK still images in your comps but, really, even that shouldn't matter these days.
You're going to have to tell us a LOT more about your projects before we can really do much for you.
Since you say it's an older Mac, I'd drop Yosemite like a hot potato and go back to Mavericks. That newer OS does NOT play nicely with AE.... for many, many users. It's just plain bad news for AE. A real dog.
Make sure your AE version is fully updated. Don't take the application's reassurances that it's updated, because it frequently misleads. Go do an update.
I see you also reserve ZERO processors for other applications. Change that to 4 processors for other applications. And while you're at it, give 8 gigs of memory to other applications instead of 6. Sounds counter-intuitive, but it frequently helps.
I suppose It's all a matter of what you want to run more smoothly: the OS or the applications.
Thank you for your replies. A little more about the project:
My maximum resolution is 720p. I have expressions riddled throughout the project. Really it is just duplicated it a bunch of comps. Not sure this affects anything. I also have things animated objects moving up and down, nothing to fancy. I have minimal camera movements in various comps but thats about it.
Dave, I changed those settings and well it is unfortunate that the OS could further complicate this program.
As I have been writing this I have checked whether the settings had fixed the issue and it looks like it has! I guess the old mac doesn't like it when AE hogs up all of the CPUs. I am not entirely sure why they even give you the option. Anyway, thank you both for your replies.