I'm contemplating getting a Mac Pro 5,1 built and I'm wondering if it will run After Effects well. Here are the specs (I'll be upgrading it as I go)
What OS are you planning to run on it?
I'm currently running Yosemite, but I'm open to whatever works best.
That should work just fine. There's a bug in AE on El Capitan where you have to hide the preview time indicator on the timeline panel (or, more easily, just make your comp panel full screen to cover it by pressing the ` key) in order to get realtime previews. However, the AE team has said they're expecting to have a fix for this bug in the next update, so I wouldn't worry too much about it if you do decide to go to El Capitan.