This post is from 2008 before the cloud really took off. It seems like an update to the answer is required. I will just provide some links and an overview. I am sure that there are more up-to-date posts at this site on this topic, and if I find them, then I will add the links here.
When it comes to data scalability and transaction processing scalability, in 2017 we need to be talk about the Cloud and Cloud Service Providers.
I think the top three Cloud Providers these days are:
Cost
One of the great thing about using cloud services is that there are no upfront costs, no termination fees, and you pay only for what you use. (Quoting Mr.Alba's 2016 article "A Side-by-Side Comparison of AWS, Google Cloud and Azure")
We use AWS ourselves. We pay only while we have VMs installed and running, so it can be a cheap way to start up. Typically, service providers charge by the minute or by the hour but you are guaranteed to have it for that entire time.
A cheaper way to go is best-effort spot pricing. The Spot price represents the price above which you have to bid to guarantee that a single Spot request is fulfilled. When your bid price is above the Spot price, Amazon EC2 launches your Spot instance, and when the Spot price rises above your bid price, Amazon EC2 terminates your Spot instance. (Shamelessly quoting Amazon's User Guide here)
A Side-by-Side Comparison of AWS, Google Cloud and Azure is a good article doing a side-by-side comparison of these three service providers available here.
For a more academic look at cloud services, read the 2010 paper by Yu, Wang, Ren, and Lou "Achieving Secure, Scalable, and Fine-grained Data Access Control in Cloud Computing" in the INFOCOM 2010 Proceedings available here, but you may need to be an IEEE member to gain access to it. While it is somewhat dated, it is excellent and you can use it as a jumping off point.
Scaling in the cloud has been exploding, and until recently that scaling was done by starting up new Virtual Machines which tooks seconds, but with Containers one can spin up new instances in milliseconds. For more information on this, check out Docker and Docker Containers here.
I apologize for this answer being just a bunch of links for more information, but I thought the answer to this question should have an update. I hope this inspires someone to provide more first hand details. If you have already posted some related information, please consider providing links to your own posts. Thanks!