I don't know about the specific overhead of rails. But if you had a uniform distribution of 2000 requests a second, and each request took up to 50ms, then you'd only have 100 requests at any given time. So each request could take up to 5mb, which is generous unless you're doing something explicitly fat.
You mean 5 mb? I can't say I have a ton of experience. But of the three projects I've worked on, the fattest was a Vaadin CRUD app, which stores the user session/state on the server and it could be around 5mb. More context: This isn't counting any DB effort.
