Kano, do you have memcached running on your site?  how much ram is there?  My strandvision site has a home built ddos system using memcached (so it is fast) that auto slows all non important requests if the server load gets too high.  It also counts hits from each class c ip address range.  If it exceeds the max per minute hits, then the system instantly dies (for that ip address) for a day.  If they do not visit again for a minute, the memory is released.  I do have white lists for paying customers to prevent issues.  Anyway it sounds like you do not have issues with it, but if you do, this type of system does not take long to implement.  PM me if you would like details.