new relic is a cloud offering, that help monitor and graph your server internals.

You can get web response times, broken down into internet/database/php times, etc

I am using this for a few other customers

new relic is free – but they limit functionality, and only keep 24 hours of data – but that is still very useful

It gives you graphs of application performance, php, web server, mysql server. It can trace transactions, show you slow pages, and slow queries etc etc.

Have a read about what they do – the video is cool
http://newrelic.com/

there are plugins for wordpress, magento, and w3totalcache has support for it in the latest release

its pretty easy to install and get up and going.