If you have been in the U.S. unlawfully for more than a year, then if you left the U.S. you will be subject to a harsh 10-year bar for re-entering the country. This is true regardless of whether you may marry a U.S. Citizen while abroad.
Generally, the most efficient process to take is to file an I-130/I-485 applicaiton to adjust status promptly after becoming married to the U.S. Citizen in the U.S. In that type of application, the fact that you long overstayed your visitor's visa will not stand in the way of adjusting status to become a Lawful Permanent Resident (to get a "Green Card").
[Note: Consistent with Avvo policy, this communication is intended as general information and not legal advice, and this communicaiton does not create an attorney-client relationship.]
David N. Soloway
Frazier, Soloway & Poorak, PC
1800 Century Place, Suite 100
Atlanta, Georgia 30345 www.fspklaw.com
404/320-7000 1-877-232-5352 firstname.lastname@example.org