[CarlitoBrigante]

You would need a very fast admin working throughout the time of the attack on your server to effectively limit the effects of the attack on the machine.

Mod evasive might also help, and even some software firewalls like APF have some auto-detection mechanism that can stop a small percentage of the attacks. Both are very easy to install, and free - but again, if the attacker knows what he is doing, they won't help much.

http://www.zdziarski.com/projects/mod_evasive/ (recommend reading: http://www.eth0.us/mod_evasive)
http://www.rfxn.com/?page_id=44

Mod Evasive can theoretically be left always on in your server, even after the attack, even though it must be carefully configured or it will stop legitimate connections and thus cut your traffic and annoy your users.