1) You could start filing 90 days before it expires (90 days before March 29, 2013).
2) You can travel without a problem.
Law Office of Luis A. Guerra (954) 434-5800. This answer is of a general nature and should not be relied upon as final, nor is it intended as legal advice.
You can submit it any time within the 90 days prior to the card's expiration.
You should retain an experienced immigration lawyer to review all the facts and advise you accordingly.
J Charles Ferrari Eng & Nishimura 213.622.2255 The statement above is general in nature and does not constitute legal advice, as not all the facts are known. You should retain an attorney to review all the facts specific to your case in order to receive advise specific to your case. The statement above does not create an attorney/client relationship. Answers on Avvo can only be general ones, as specific answers would require knowledge of all the facts. As such, they may or may not apply to the question.
The law requires a person to the I-751 to file within the 90 day period prior to the expiration with a few exceptions. Whether you are okay to travel depends on whether you remain admissible or otherwise impacted your eligiblity for conditional permanent residency. I agree you should consult with an experienced immigration attorney prior to any travel outside the US to avoid problems when trying to seek permission to re-enter.