Is there a simple way how I could redirect a visitor (via .htaccess or PHP script) to a static page when the server is overloaded from too many requests?

It doesn't have to be a protection against huge amounts of requests at once or protect against DoS Attacks. I think our server would be protected enough if we could prevent the standard website to be shown and instead show a single file "overloaded.html".

Also how could I get a measure for a server being overloaded on a typical managed server (= non root access to a Linux server) environment?