First public deployment coming up. How to load test?

So I’ve been working on a project for a while with some friends, and we’re getting ready to launch the beta.

I’ve never built a website on this scale, especially not in Rails, and in the two weeks I have to prepare I need to figure out how to predict and optimize for when traffic (hopefully) hits.

What sort of tools are commonly used for load testing and analytics? How can I predict what I’ll need in two weeks, and what should I do then if we’re lucky enough to be pounded by traffic? How much traffic do you think a 256MB Slicehost slice with Passenger behind Apache could support? Should I use Rails’s built-in caching?

Thanks! Any advice, even if anecdotal, is welcome.

siege - if you have time but not money -

Soasta - if you have money but not time -