Bottom line is that there is no easy way to do it.
Most companies doing it use some kind of custom, proprietary, or third-party solution, to use as a combination load-balancer/failover director.
If you want to build one you can do the analysis.
One thing to think about would be two servers, each running DNS pointing to it's own box (I've discussed this on these forums before), and for sites requiring MySQL, MySQL on yet a third server, or perhaps running some kind of mirrored MySQL solution. Onno and I looked at one a few years ago but were never happy with it.
We'd be happy to take on the analysis as an open-ended consulting job, but I couldn't possibly know how much time we'll spend before we have a solution for DirectAdmin, and in advance I don't even know how many servers would be required or what the ongoing costs might be in terms of a percentage of the cost of a single server.
Perhaps someone else who posts here might already be experienced.
Anyone?
Jeff