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There are discussions of this on a number of web sites on the net containing explanations from immigration lawyers. I am not one and I urge you to look for information at imminfo.com, immigration.com, shusterman.com, isn.org and other fine sites on the net.
As I understand it, the green card is for a job in the future. When you get the green card you must have the intention to work for the sponsoring company for an indefinite period of time. Of course things can change between the moment you get the green card and the moment you decide to leave. If the period of time between these 2 momemts is very short you may have problems when/if the INS decides to come after you - if that ever happens.
Of course from a legal point of view you posting this question on the net shows intention to leave your current job ...
As I said just my personal opinion.