You can get away with following oral(blog)-history scaling lore at that low end, but before you scale much further the most important thing by far is to be more

You can get away with following oral(blog)-history scaling lore at that low end, but before you scale much further the most important thing by far is to be more metrics and measurement oriented. The most basic one to start with is average http response time and http requests per second. For instance, say your slicehost returned a page at an average of 200ms, but only below 10 req/second, over that it started to bottleneck somewhere and responses would shoot up to 500+ms. Then your question is "how to I handle 100 http requests per second while keeping the average at or below 200ms". Eventually you'll need to step up to full-browser-render (gomez, keynote, browsermob, homegrown-selenium) time, appdex or 95th% based numbers instead of simple averages, and backend components like mysql req/s and service time. The point is, this is engineering, use numbers.