It will go along way to remain calm and remain acting like an adult throughout the conversation. I would approach them in a way as if you are coming to them for advice on the situation, not "telling them how its gonna be". They will likely say no (I don't know your parents, but mine resemble very much like what you described, so I'm going off that). State how you feel about the situation, the pros and cons that you see, possible outcomes, etc., etc.. This will show a maturity, and not just a "I want to get the fuck outta here" attitude. Once they see that you are not just blindly rushing into something just to get out from under their thumb, but making a thought-out rational decision, their own attitudes will be different. I'm not saying this is going to make them jump on board, but it may soften the blow when you do tell them of the choice you have made. There may not be the big falling out, and the "no-more-college" bullshit that there would have been.
Also, don't feel that the whole issue has to be resolved in a single sit-down conversation. You can bring it up, and if things start leaning towards an argument, let it go for a few days before it becomes an issue. They may realize that although you are their child, they can't hold onto you forever, and this is merely the first big step in that direction.
I don't know, much of this stuff is hard to give advice on, since everyone's relationship with their family is unique, but I hope that what I said helps you somehow.