Rackspace offers a multitude of hosting options and in my opinion is one of the more flexible hosting options in the market place today. They offer both managed and self-managed options; I have only utilized Rackspace for self managed hosting.
With cloud hosting you can add or removed servers on-demand. Each server can be of any flavor you’d like, meaning you can pick the CPU, RAM, storage size, and bandwidth you need. Servers are paid for by the hour, so you can spin one up for testing something quickly and have minimal charges. Additional storage for images, media, etc. can be added as block storage to meet any of the capacity you might need.
In addition, a load balancer can be utilized paired with an auto-scaling server configuration. With this configuration, as website traffic increases and demand requires new servers can be added automatically to ensure every visitor has an optimal experience.
One other point I’d like to bring up is the monitoring offered by Rackspace. You can keep tabs on the CPU, RAM, HDD, TCP traffic, etc. in realtime monitoring. This can be great for troubleshooting code issues, server problems, traffic issues, etc.
After using Rackspace for several years now, I would recommend their services for websites both large and small. I encourage you to take a look at what they offer and give them a try. Like I said, their billing is hourly, so if you don’t find it suits your needs, the expense for testing it out will be minimal.