An interesting thing is that Isaiah is made up of 66 chapters (edit: I'm not saying that has any relation to the number 666), which is the same as the number of books in the Bible. Moreover, Isaiah is clearly divided into two sections: the first 39 and last 27 chapters. This is the same as the number of books in the old and new testaments.
I'd suggest, though, that a more accurate place to start in interpreting Revelation 13:18 would be in old testament passages where the number 6 is emphasised. Solomon, during the latter part of his reign, disobeyed Moses' commands not to accumulate wealth. We're told he had an anual income of 666 talents of gold. There is also the dimensions of the image of gold set up by Nebuchadnezzar: 60 cubits high and 6 cubits wide. Also note some of the measurements given for the Philistine giants, such as Goliath and the four others mentioned.