I entered usa in December 2004 as an f1 student and would start a job on opt in September 2009. Is it true that based on 5 calender years I am a resident alien for the year 2008.

If so, would it make any difference in my taxes (fica/social security/medicare) if I start my h1b in October or should I remain on opt till next October.

In 2009, your income is subject to SS tax and Medicare tax. You will file resident tax return, and can not claim any treaty deduction.

The key is what you INTENT will be after the OPT period.

If you intend to apply for a H-1B visa, then you cannot claim "a closer connection" to your home country in order to retain your non-resident status in order to avoid paying FICA taxes. Based on your posting, this seems to be the case.