What you probably need is Selenium RC and Selenium Grid as Silenium IDE is quite limited on automated testing. RC allows you to run remote selenium tests (tho rc can run locally too) and grid allows you to simplify the access to all running rcs.
You need 40 clients at once. If you are using selenium-rc you can start severall clients simultaniously by configuring them to run on different ports. After that you have to start your test 40 times at once. That is the tricky part depending on what framework you are using to launch the tests.
I would suggest JMeter for load-test like situations. It is quite easy to setup and you can configure how many simulated users you want on your website at once. JMeter works fine for manuell tests and for automated tests.