This is definitely not a free solution, but webmetrics will get the job done. (As well as any website monitoring company in general). They give thorough reports on your web site's performance, and when you schedule load tests you can specify how much load you want to simulate (e.g. simulate 2000 users using my site at once).
I don't think it will break it down and tell you your CPU usage and such, as they stress your website from their servers.
