First off you need to know if you think you can make that much, or more.
After that, it would really do you some good to put together a business plan of some sort, one that identifies your target market, the services you aim to provide, as well as the costs you will incur, the funds needed, and the pricing for your services. It will help to know your compe ors' pricing. Will you compete on price, service, or both? How will you acquire clients? Do you have your contacts organized? Depending on your situation, you may want to start moonlighting first before you dive headfirst into a consulting business without any clients. Find a template for a business plan and fill as much as you can out. It would do you some good to put your plan into writing and flesh out the detail. If you are planning on doing this full-time, do you have enough saved up to survive when work dries up for a month or three? Also, keep in mind that your pay arrangement is no longer routine, you get paid when the client so chooses.
I can tell you now that consulting is somewhat challenging, due in no small part to the client relationship. Whereas an employee can sit around and jack off all day as a consultant you cannot do that. As a vendor you will encounter more scrutiny. Availability is a big issue as well. Another is communication and clarity on the task and deadlines.
The last thing I will say is that realize that the buck stops with you in running your own thing. As an employee all you have to worry about are your work and keeping the boss happy. As the boss, in addition to the work, you have to worry about the client relationship, managing your receivables, cash, paying your employees, your legal structure, keeping your employees happy, and so on.
Anyways, best of luck. It's good that you are thinking about striking out on your own.
-HC