I think - If you graduate in December, and take your OPT effective from December, you will be able to utilize your OPT as well as apply for H1B. With masters degree, August may be also fine as there might be Advanced Degree Quota may remained until August but still December w/ OPT is on safe side. New QUota for 2008 strats from APril. For MAsters Degree - there are still 11000 quota remained for 2007 but dont know if it will last until August.