If you read Hebrews 7 that exact verse is described and explained for you. Melchisedec is from what I understand a pre existant christ, just like in Daniel where one like unto the son of God is seen in the fire. Dan 3:25. He is a priest that has no beginning or end, which I would understand to be Jesus 

You're right, Melchizedek is NOT from the line of Aaron. He precedes Aaron. Melchizedek foreshadows Christ and may even BE Christ pre-incarnate.[Psalm 110:4] is a prophecy concerning Christ wich says that Jesus is "a priest for ever after the order of Melchizedek". Melchizedek is a different order of priest than Aaron. Aaron's line was temporary...meant to last only until Jesus fulfilled the Law and atoned for our sins once and for all. Melchizedek's line seems to be eternal [Psalm 110:4]. If you read [Gen. 14:20] in the New International Version, you see that it was Abraham who gave Melchizedek the tithe. Pre-incarnate Christ or not, the feeling I get from these passages is that Melchizedek was no ordinary priest.