Cisco WAN :: 2621 / 3600 / 5505 - Work With BT Infinity

We have an existing Cisco 2621 XM router with an ADSL interface connected to BT Broadband.  Basic layout below: 
Internet------ADSL on Cisco 2621 ------ Pix 506e ---- 3600 switch with inter VLAN routing to multiple VLANS
 
There are 5 static IP addresses.  The internal FE port on the 2621 has one external assigned to it.  The outside int of the PIX has another external IP assigned from the 5.  The other 3 are NATd through to internal IPs via the PIX to internal servers etc.
 
The ADSL is being upgraded to BT Infinity.  This new service comes with a separate VDSL modem and a separate BT router.  Ideally I want to ignore the BT router and use the Cisco 2621 as it has 2 FE ports and I suspect I could do something using PPPOE. 
 
So New layout would look like 
Internet ----- BT VDSL modem------ Cisco 2621 ----- Pix 506e ---- 3600 switch with interVLAN routing to multiple VLANS
 
I think it is a bit strange the way it is setup as 2 static IPs cant be used.  There is an opportunity to use a ASA 5505 which may become available.