Yeah, the whole deal in Acts, Peter's sermon, was special, it was pentecost, the Jews were just receiving the Holy Spirit. Then later in Acts the Samaritans, the disciples of John the Baptist, and the gentiles recieved the Holy Spirit. Now when one believes, they are baptised into the body of Christ by the Holy Spirit, water baptism being an outward profession of that faith, and a symbol of the death, burial, and ressurection of Christ, and wonderful experience in obedience to the Lord.
Once a person has accepted Christ, at the moment one believes, they are saved. Everything after that is good works or bad works that are not meant for us to earn or keep our salvation but are a result of our salvation. We are saved by Christ's blood alone. So even though baptism is one of the first good works we do after we are saved, it is important to distinguish that it is something we do AFTER we our saved, not to get saved or keep saved, just like no amount of good works will save or keep you saved. God keeps us, his death covers ALL the sins of ALL the world which includes all your/my sins. So unless you want to lose your freedom in Christ, and become a legalist, like the Galatians, you should not add anything to Christ's death, by adding something else you do besides accepting the free gift, you take away from what he did. Again it was by his death, his blood, that our sins were paid for, and there is nothing we can do to add to that.