Very fuzzy. I was mostly trying to be funny---one is always getting those email quizzes about "Do You Speak Southern?" I just want to make sure my region is represented for all these English students :)
"Fixing to" can actually have a fairly broad range of possible time. You could replace it with either "getting ready to" or "going to," but in my mind at least, its meaning is closer to the former.
"We're fixing to drive the minivan out to the Grand Canyon next month, so I need to get that transmission looked at." There's still a certain sense of imminence - wow, that trip will be here before we know it, better check out the engine.
And "fixing" isn't as long-range as "going." You wouldn't say, "John is fixing to marry Nelly after he finishes his two-year Navy tour of duty."
Immediacy, of course, can be heightened with "just" -- "I'm just fixing to walk out the door, Mama. Can I call you back?"
Really, "going to" is the most versatile of all these combinations. Everything from "The sun is going to burn out, and all life on Earth will cease" to "Pull over, I'm going to throw up."