http://uscis.gov/graphics/howdoi/immvisa.htm
"What is an Immigrant Visa Number?
An immigrant is someone who is not a U.S. citizen but has been authorized to permanently live and work in the United States. If you want to become an immigrant, you must go through a three-step process."
"First, the USCIS must approve an immigrant petition for you, which is usually filed by an employer or a relative for you."
that mean your I-140 must been aproved then go to next step.
"Second, a visa number, through the State Department must be immediately available to you, even if you are already in the United States. If you receive an immigrant visa number, it means that an immigrant visa has been assigned to you."
"Third, if you are already in the United States, you may apply to adjust to permanent resident status after a visa number becomes available for you."
That mean when you file I-485 and USCIS send I-485 receipt to you, a visa number available for you.