redundant internet and services

Network Gurus, what do I need so that when our primary ISP goes out, secondary ISP will takeover without any interruption in service? This mean internet needs to be working, site to site VPN needs to work, VPN client needs to work, and inbound/outbound rules will also need to work. Basically, everything should be seamless beside a minor disconnect. How much would something like this cost and if hardware is required, would we need one in each location? Currently, we're using a SonicWALL with a simple failover configuration. Not sure if the SonicWALL can do all of this. Thanks