Dual WAN router/Cisco PIX setup for internet failover

Would like to combine two ISP’s WAN connections via a dual WAN router at our main location. Currently, one dsl connection supplies internet connectivity. We have 7 locations all connected in-network via PIX-to-PIX (site-to-site) VPNs. Two Windows 2003 Servers, an Exchange Server and appx. 25 workstations are at the main location, hence the need for dependable service. Can the router combine the connections ONLY and let the PIX 506e continue to be the VPN server and traffic-router as is is currently functioning now? Or, what setup changes are required? Thanks for any help.