I am too busy. I am over the amount of busy-ness that would allow me to talk to you. I have more things to do than I can handle while I am talking to you at the same time.
I am busy enough. I am busy in the correct amount. This doesn't make much sense.
It indicates that I can't talk to you until I am busy at the right level. If I'm not busy at all, I can't talk to you. I have to be doing at least one other thing while I am talking to you.