as much as I hate to cut and paste:

Acts 10:44-48 While Peter yet spake these words, the Holy Ghost fell on all them which heard the word. 45 And they of the circumcision which believed were astonished, as many as came with Peter, because that on the Gentiles also was poured out the gift of the Holy Ghost. 46 For they heard them speak with tongues, and magnify God. Then answered Peter, 47 Can any man forbid water, that these should not be baptized, which have received the Holy Ghost as well as we? 48 And he commanded them to be baptized in the name of the Lord. Then prayed they him to tarry certain days.

Notice that the Jews were astonished by the sign, that Peter used the sign as a basis for baptizing Gentiles, and that Peter commanded them to be baptized in the name of the Lord. This was the baptism of Acts 2:38 based upon which those baptized received “the gift of the Holy Spirit.”

Look also at Acts 11:14-15. Verse 14 tells us that Cornelius would hear words whereby he and his household would be saved, and verse 15 tells us that the Holy Spirit fell on them as Peter began to speak. So the Holy Spirit fell on them before they heard the words whereby they would be saved, and certainly before they could obey whatever Peter told them to do. So we know that the Holy Spirit fell on them prior to their salvation.

So why then did the Holy Spirit fall on them? It was not for their benefit but for Peter's! Look at Acts 11:16-17. When Peter saw the Holy Spirit fall on the Gentiles he was reminded of something Jesus had said, and he knew that the gospel was for the Gentiles as well as for the Jews. Look also at Acts 10:45.

And what did Peter tell them to do whereby they would be saved? Look at Acts 10:48.