A more slightly more sophisticated approach would involve having a process or cron job that polls the machines periodically and decides which machine(s) can handle more load.

BIND 8's nsupdate facility was originally intended to make interoperability with DHCP a little more seamless. However, one of the side effects is that, by giving us the ability to dynamically update a zone, we can have a machine's preference in the zone be:

favored

it can handle more load

disfavored

it has met or exceeded it's capabilities

eliminated

the machine is taken off out of the rotation for maintainence