The IT department of the nonprofit organization I work for recently got a new virtual server running CentOS (with Apache and PHP 5), which is supposed to host our website. During the process of setting up the server I discovered that the slightest use of the new machine caused major performance problems (I couldn't extract tarballs without bringing it to a halt). After several weeks of casting about in the dark by tech support, it now appears to be working fine, but I'm still nervous about moving the main site there.

I have no budget to work with (so no software or services that require money), although due to recent cut backs I have several older desktops that I could use if it helps. The site doesn't need to withstand massive amounts of traffic (it's a Drupal site just a few thousand visitors a day), but I would like to put it through a bit of it paces before moving the main site over.

What are cheap tools that I can use to get a sense if the server can withstand even low levels of traffic? I'm not looking to test the site itself yet, just fundamental operation of the server.