I've had quite alot of experience building and installing HA load balancers using Linux variants, but I've yet to do so using BSD. HAProxy has FreeBSD and OpenBSD sources available, though, and is very reliable - so you might want to give it a shot. If you're interested in a Linux solution there's also IPVS.
Either way I suggest using a pair of load balancers in a failover configuration using sysutils/heartbeat (if using BSD).
For several of my production clusters I've used 4 identical machines where 3 of them are web servers (which are almost always FreeBSD) that are served to round-robin and the 4th is the master MySQL server (also usually FreeBSD). Typically I set up one of the web servers as the MySQL slave using replication.
And, in some cases, I've mounted the /www directory to each server from the MySQL server, so I can add new web servers to handle additional traffic.
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