If this is a webapp (and from your 300 user request I'm guessing it
is), ruby is not currently be the right way to test.  The load/stress
testing components available for Ruby are less than complete.

Take a look at Jmeter (http://jakarta.apache.org/jmeter/).  It will
probably do what you want it to do, and most likely will only take you
between one afternoon to two days to complete a good suite of tests.

Don't give into the temptation to try and use Watir or some other
functional testing system for load/stress testing.  They aren't
designed for it; getting them to work in this situation will be an
exercise in futility; any results will be dubious at best.  I'm
working as a test developer, and Jmeter is my tool of choice for
load/stress testing of web applications.

As with any testing, a little common sense will go a long way, and
using common sense won't be completely natural on the first attempt ;)
I know this from experience.

Give a holler here, or on the jmeter-mailing list if you need help.
They are very responsive and helpful (just like the ruby list!)

--Kyle