Do I collect at age 66 or age 70?

I work full time with a very nice annual income and do not plan to retire for many years to come. I will reach the age of 66 in February of 2011 which is when I can start collecting at the 100% level. My dilemma is do I start collecting at age 66 or wait the four years when I am 70? I ask, as the difference in my monthly check is $800