I personally use Rackspace as well. But pay attention when you go there, they offer 3 different products:
  • Cloud Sites
  • Cloud Files
  • Cloud Servers

You want the "Cloud Sites" - it took me quite a conversation with the Rackspace Chat Team to get confirmation that this is the only of their products that offers unlimited growth, so you will never have to migrate to a bigger server or anything like this any more.

It's tempting to gor for Cloud Servers, but they are basically like physical dedicated servers which can be maxed out.

When you go for "Cloud Sites" expect a short learning curve with their admin interface - it's NOT cPanel, but a Rackspace special one. It offers far less options than cPanel (e.g. in terms of email addresses management), but up to now I didn't really miss anything (maybe apart from the fact that minimum cron job repetition time in 5 minutes, so your cron jobs won't run every single minute).

The technical support via chat is excellent, so I can give Rackspace Cloud Sites a real 'thumbs up'.