[007c]

Hi would anyone know about any software or scripts that could test out my website's maximum unique visitors before making it crash or slowing it to a crawl?
Atm i'm on a shared server so i'd find it interesting to see how much my site can handle with current php scripting and sql that's on it.
For ex sometimes if google and yahoo are crawling my site at the same time it can crash it for a few mins as there's alot of sql queries and pictures being requested.