Consider using IPVS

[dasher]

You might want to consider using IPVS as a load balancing and performance management control.
It allows you to implement servers behind the IPVS host that are transparent to users - and allows you to bring servers on & offline without affecting (or even without them knowing) users.
Additionally it can perform performance management by controlling the load hitting a single server.  You can implement rules that will shift the user to another (less loaded) machine or back them off until resources become available.

It's a really flexibile solution - esp when combined with virtual servers acting as front-end web servers.  A decent virtual hosting company will even provide interfaces that allow you to alter the v-server config on-the-fly (allowing more machines to be brought online to allow scaling out, or assigning more resources to v-server instances to allow scaling up) - which when tied to IPVS allows you to respond in a very dynamic way to user load.