I know this topic has come up a million or so times on this site, but I couldn't think of a better place for advice on a web application that is in need of hosting soon.

I've used Liquidweb's dedicated servers for most of my previous applications. They're powerful, come configured ready to go (apache, php, mysql, mail, panel, etc), and the support is by far the best. My main concern now, is scaling, server power, reliability, etc (who's isn't, right?). The webapp is built on Drupal. When clients sign up, they automatically get a private copy of the application, available via a subdomain (single codebase, separate DB). Because of that sort of system, and my desire for a massive amount of clients, scaling and server resources are a big issue (having a hundred or so subsites operating together, big cron jobs, indexing, etc). I first considered getting two dedicated, one for DB, and one for everything else. But Amazon's, GoGrids, and Rackspaces cloud offerings seem worthy of a try.

I have no experience using any cloud-like services so I don't really know what to expect. What worries me a bit, is that I don't want to have to install and configure everything needed by the servers (mail, web, db, panel, etc). I'm pretty sure none of these services come preconfigured like that, and I don't know how well any of their support is (and monitoring is probably non-existent). They also seem a little expensive to be running 24/7/365. Most of the wording on the site place emphasis on the service being for temporary server power, computation, etc.

I greatly appreciate any feedback, thoughts, advice, etc..

Thanks as always