this really doesnt do you any good. If Megapath breaks on the WAN side of the router, your PIX will not failover.
you need a segment inbetween, a switch, that holds both routers and both PIX's and then OSPF and HSRP between the two routers.
then you need to address the public IP range side of this. you need a Provider Indepeendant Range - a PI range aasigned to you. next you need to arrange the BGP with the two providers. and keep in mind the slow convergence times of BGP.
IP space is almost immaterial, as long as both ISPs agree to advertise it and you have an ASN. Basic design is to have one router peer (EBGP) with Qwest, the other peer (EBGP) with Megapath, run IBGP between them, run HSRP between them for the PIX to use for default GW, configure PIX to be default GW for interior users. That eliminates PIX, routers, lines, and ISPs as single points of failures. Does not eliminate switches or configuration errors as a single point of failure. From the viewpoint of the design, the PIX and the failover PIX are a single box with a higher availability (and some weird failure modes, but that is a different can of worms).
Set up details for BGP will depend upon relative importance of fail over versus load balancing versus budget. Other approaches are possible, depending upon your specific requirements.