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.
Raph

siege - if you have time but not money -
http://www.joedog.org/index/siege-manual

Soasta - if you have money but not time - http://soasta.com

Cheers,

m