If you are added to her application now before her 485 approval, you are her derivative and you will NOT be considered 婚姻綠卡. It is still employment-based green card application, you are just deriving the benefit because of your relationship to her.
If you marry after her GC, then it will be considered 婚姻綠卡, and as a result, your priority date will be the date of your filing. 
one more clarification
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                    thank you also!I have same
                     -question!- 
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                        06/07/2005 postreply
                                                16:39:51