It's best to focus on the end points and beef up security there. Focus primary security controls on the application and not the perimeter. One of my biggest frustrations as a security professional is walking into an environment where systems which must be highly secure are accessed via simple username & password. All banking applications at a minimum should require x.509 client auth for employees utilizing a private-key stored on a device which is not permanently attached to the system. Monitoring solutions should then be in place to track authentication actions and provide that visibility to security staff and the employee's themselves. That's a pretty basic first step and one I rarely see in practice. Next, rather than isolating networks, start paying attention to the traffic on the networks & limit transactions to known good entities. After that organizations need to consider their customer environment security and how they may be inadvertently compromising it. It's amazing how many times I've gone to a public facing banking portal and spotted third-party JavaScript loaded within the same origin context of an authentication form. One bank I looked at awhile back actually had an advertisement from a third-party ad network on a page where they asked for credentials! That's pretty much asking for their customers and thus their accounts to be compromised.
