If you are considering cloud computing, then I guess you have worked out the reach of your application. If you haven't (i.e. you don't know what's the user grow

If you are considering cloud computing, then I guess you have worked out the reach of your application. If you haven't (i.e. you don't know what's the user growth rate), then you don't need cloud computing (yet). Just get a VPS with Slicehost or Linode to start with.