I am planning to give my Gmat exam by Dec 30 2014 and want to take admission for MBA in fall. Am i late in giving the Gmat exam?

No. The earlier the better, of course, especially if you need financial aid. I can't think of any schools that close their admissions so early for their MBAs.

I gave my gmat today and got a score of 450. Planning to reappear in december with better preparation in verbal section. I am worried of being late for admissions to business schools in USA. Should i try to apply with this score (which is low) to any colleges for now?

why don't you opt for an online MBA degree? you can save you GMAT time and money aswell as you would not have to worry about getting a 700+ perfect GMAT score to get into a good college. Online education previously was seen as a total scam but today there are employers who value a MBA earned online because it is the same course and skills as a traditional one. A U.S. research concluded last year that there were 7.1 million students who took an online degree.
Read more on research : http://www.babson.edu/news-events/babson-news/pages/140115-babson-survey-study-of-online-learning.aspx.
Read more on online MBA: http://www.usnews.com/education/online-education/mba

I beg to differ: online MBA programs are great, but they're not for everybody. Many people take full-time MBA programs because they want a significant salary increase, or they're looking to make large functional or industry shifts. Plus, you can't build your soft skills to the same extent in online programs.

These two different types of programs serve two vastly different functions.