The literal meaning of Now that's what I'm talking about is along the lines of: in the middle of a conversation about some topic, with examples being given, someone or something comes into view that typifies the thing being spoken of, and the speaker draws attention to it.
That's what I'm saying is fairly close in meaning, but rather than a third party person or object, they are referring to something that the second person said.
I'm a little stuck for an example, but let's say we're talking about tiny dogs wrapped up in coats against the winter:
A: I saw a tiny dog walk past the other day in this little coat, looked just like a burrito ...
B: [looks puzzled]
[small dog walks into view, wrapped up in tartan, looking like a burrito]
A: ... that's what I'm talking about.
A: I can't help but laugh when I see those little dogs with tiny legs wrapped up like hotdogs in little tartan coats ...
B: They look just like burritos!
A: Yeah, that's what I'm saying
Not actually a very good example (sorry!). Maybe watch The Fresh Prince of Bel-Air (Wikipedia) - Will Smith uses at least the first of those phrases many times.