How to manage enterprise network of Linux machines?

As previous answers say, you have very powerful tools like Puppet, chef, cfengine for advanced configuration management.

But if you want a tool that can do installations, easily manage configurations, deploy systems from templates, apply updates, or remotely execute custom scripts... you can try an integrated solution like SpaceWalk, which is the open-sourced version of the RedHat Satellite. Note that it can work with other distributions, not only Redhat!

I've personally used the commercial version to manage more than 1000 hosts.

The keyword you are looking for is configuration management. This is provided by several tools: Chef, Puppet or CFEngine, for example.

With those tools you can divide your servers to groups, and then perform tasks to single servers, single groups, or several different servers / groups, or all the servers simultaneously. Tasks like "Add package X to group www-servers", "Change /etc/resolv.conf DNS servers from group database servers", whatever you need to do, will be more trivial to perform after the initial shock you encounter due the sheer amount of things you can do with CoMa software.

For user account management I say use the LDAP you already have, Linux fully supports that and is about the only sane way to do anything in a bigger environment.

One more alternative: you did not mention what distro you use, but if it happens to be Red Hat Enterprise Linux, you can use their Red Hat Network for managing software upgrades in a more granular way, among other things it can do.