If I have understood your requirements correctly, you might not need to buy another PIX. You could have the following topology:
Inside Network -- PIX -- 1700 -- Internet
The 1700 could be configured for PPPoE and for bringing up the backup interface when necessary. All the traffic, regardless of which link the 1700 is sending it over, will pass through the PIX.
You could refer to http://www.cisco.com/en/US/tech/tk175/tk15/technologies_configuration_example09186a0080093fbf.shtml for more information on configuring 1700 as PPPoE client with NAT.