Thought I'd just send my 0.5€ worth of info: mod_cband, as stated in
the previous comment, is exactly what you need.

Of course, if you have some sort of Linux firewall/balancer or your
web server is running in Linux, you can get away with iptables and the
limit module (max connections per second, for instance), which is way
faster then letting the connections go all the way up, just to be
blocked anyway.

Regards,
Ricardo Oliveira
http://apache.superbofh.org/ - Apache book in Portuguese