Apache'de ip başına bağlantı sayısını limitleyen modül kurmalarını isteyebilirsin.
Edit: Biraz daha ayrıntı vermek istedim. Mysql'den önce Apache tarafında mod_evasive (
http://www.nuclearelephant.com/projects/mod_evasive/) ve mod_limitipconn kurmalarını sağlarsan sanırım sunucu yükün normal seviyelere inecektir.
Ayrıca httpd.conf ayarların çok önemli.
Aşağıda şuan kullandığımız epey yoğun bir sunucunun httpd.conf dosyasını görebilirsin.
#
# Timeout: The number of seconds before receives and sends time out.
#
Timeout 300
#
# KeepAlive: Whether or not to allow persistent connections (more than
# one request per connection). Set to "Off" to deactivate.
#
KeepAlive Off
#
# MaxKeepAliveRequests: The maximum number of requests to allow
# during a persistent connection. Set to 0 to allow an unlimited amount.
# We recommend you leave this number high, for maximum performance.
#
MaxKeepAliveRequests 100
KeepAliveTimeout 15
# MaxRequestsPerChild: maximum number of requests a server process serves
<IfModule prefork.c>
StartServers 20
MinSpareServers 30
MaxSpareServers 50
MaxClients 150
MaxRequestsPerChild 250
</IfModule>
# MaxRequestsPerChild: maximum number of requests a server process serves
<IfModule worker.c>
StartServers 2
MaxClients 150
MinSpareThreads 25
MaxSpareThreads 75
ThreadsPerChild 25
MaxRequestsPerChild 0
</IfModule>
# perchild MPM
# NumServers: constant number of server processes
# StartThreads: initial number of worker threads in each server process
# MinSpareThreads: minimum number of worker threads which are kept spare
# MaxSpareThreads: maximum number of worker threads which are kept spare
# MaxThreadsPerChild: maximum number of worker threads in each server process
# MaxRequestsPerChild: maximum number of connections per server process
<IfModule perchild.c>
NumServers 5
StartThreads 5
MinSpareThreads 5
MaxSpareThreads 10
MaxThreadsPerChild 20
MaxRequestsPerChild 0
</IfModule>
--------------------------------------------------------------------------------------
top - 17:03:36 up 1 day, 23:43, 2 users, load average: 1.48, 1.87, 2.15
Tasks: 213 total, 1 running, 212 sleeping, 0 stopped, 0 zombie
Cpu(s): 24.3% us, 10.0% sy, 0.0% ni, 62.0% id, 3.0% wa, 0.2% hi, 0.5% si
Mem: 2069564k total, 1869344k used, 200220k free, 67440k buffers
Swap: 4096532k total, 512k used, 4096020k free, 892148k cached
Kısaca apache'yi iyi optimize edersen sanırım mysql ile uğraşmana pek gerek kalmayacaktır.