talisker and wybnormal have already covered the default floating static very well. This is a really unconventional solution, but if these are the only pieces you have to work with, then so be it. BTW, Remember, there are many ways to skin a cat in the Cisco IOS.
I am assuming that your PIX has at least two interfaces and the 3660 has one ethernet and two WAN interfaces. Put one of the interfaces at security level 0(outside)and one at security level 100(inside). The "outside" IPs should be your primary ISP. Setup one global NAT pool and static nats as needed for the primary ISP.(Mental Note-DNS needs to be considered if you have services that need to be accessed inbound-wise). Then setup a nat cmd for the global to what needs to be one-to-many NATed.
Now your 3660 should have an ethernet interface and two WAN interfaces. Now setup two default routes(and floating default) like talisker and wybnormal suggested. You will need turn on RIP on the two serials(passive) and have your provider send you a default route if you need redundancy a few hops down the road.
The major dilemma is that even if the primary route goes down you still have a NAT issue. Setup NAT on the 3660s ethernet(ip nat inside) and secondary WAN interface(ip nat outside). Use your secondary ISP addressing for setting this up. Then you will NAT the already NATed traffic from the PIX when traffic uses the secondary ISP WAN interface. And the default floating route or the default route that your provider will provide(if you need multihop failover) will take care of the routing failover.
I thought of a better and very clean way to your problem using MPLS while writing this since you do have a 3660, but that might be a little to advanced for the forum. If you guys want me to write that solution, let me know.
rlluis@tampabay.rr.com. -Later.
