1.which is the earliest semester i can apply for?
Technically you could still apply in R3 for Fall 2012. A much safer & better bet would be R1 for Fall 2013.
2.do all schools have fall and spring semesters?
As far as I know, none of the top schools have spring starts. Schools typically begin only in Fall of each year. An exception to this rule at the top of my head is Columbia which has something called as a J-term since they start a batch in January.
3. will i be considered as an international student or out of state student?
International
4.which are the universities i can apply with my score?
The admission process is very holistic in nature. Your work experience, post-MBA career goals, GPA, GMAT, extracurriculars, school research and other aspects such as your leadership skills, team work experiences etc. will all be evaluated to ensure that 1) you have what it takes to be successful post MBA and 2) you will be a good fit with the school. Hence it is impossible to indicate which schools you can apply to with that score. There are folks who have gotten into the best schools of the world with that score and also hundreds of applicants who have been dinged with far better scores...
5.should i consider retaking the gmat to better my score?(as 650 is the lowest i've got in my mock tests.range-640-740 in some 25 tests)
Yes, in my opinion.
6.how helpful is having a green-card ,as we are expecting to get it in a few months( 6-8 months)?
Not helpful from an admissions perspective. Might be helpful post-admission if you're looking to take up a loan..Am not entirely sure about this loan part though.
I would recommend that you create an account on any school's online application portal even if you're not planning to submit an application this year. Have a look at the kind of information that is asked for and that will be a good starting point for your introspection regarding an
MBA and beyond.