Amazon has a service called EC2 (Elastic Compute Cloud) that is PPP (pay per processor — my term). It's great; it's cheap. However, you're screwed with serious traffic surges. So… Elastic Beanstalk (EBS) to the rescue!
It essentially runs a cluster of EC2 instances, and ramps up computing power as required. Never pay for more than you use, but always have shit-tons at hand when necessary.