ISA is one of the options. Others include firewall VPN appliance boxes like Netscreen or Cisco. Open source alternatives include Smoothwall. You will find that your network connection uptime is dependent on the tunnel between the sites as well as your provider(s) network(s) and last-mile reliability. Rarely is there a network provider problem (knock on wood)… it is usually a last-mile local carrier problem that takes a network link down. The VPN tunnel is only as reliable as the solution you put into place and how familiar you are with the gear. If you have an implementation partner, then that will be in your favor to keep this link up as required.