Another quick bit about what is supported with a single NIC ISA server in terms of VPN

as per unsupported article http://technet.microsoft.com/en-us/library/cc302678.aspx

"Site-to-site virtual private networks (VPNs) are not supported in a single network adapter scenario. Remote client VPN access is supported in a single network adapter scenario"

 

i.e. Remote client VPN would still work with it.