[CentOS] Apache Server Tuning for Performance

Hi all,

I am facing  facing performance issues with our web servers which is
working for concurrent  250 requests properly and then stops
responding when the requests are more than 250 .

The current configuration parameters are as follows :

apachectl -version
Server version: Apache/2.0.52
Server built:   Jan 30 2007 09:56:16

Kernel : 2.6.9-55.ELsmp #1 SMP Fri Apr 20 16:36:54 EDT 2007 x86_64
x86_64 x86_64 GNU/Linux

Server Hardware :
MAIN MEMORY
(i) Memory Size 4 GB  Dual-Core Intel 5160 processors.

httpd.conf

Timeout 300
KeepAlive On
MaxKeepAliveRequests 1000
KeepAliveTimeout 150

## Server-Pool Size Regulation (MPM specific)
# prefork MPM

# StartServers: number of server processes to start
# MinSpareServers: minimum number of server processes which are kept spare
# MaxSpareServers: maximum number of server processes which are kept spare
# ServerLimit: maximum value for MaxClients for the lifetime of the server
# MaxClients: maximum number of server processes allowed to start
# MaxRequestsPerChild: maximum number of requests a server process serves

<IfModule prefork.c>

StartServers       8
MinSpareServers    5
MaxSpareServers   20
ServerLimit      251
MaxClients       251
MaxRequestsPerChild  4000
</IfModule>

# worker MPM
# StartServers: initial number of server processes to start
# MaxClients: maximum number of simultaneous client connections
# MinSpareThreads: minimum number of worker threads which are kept spare
# MaxSpareThreads: maximum number of worker threads which are kept spare
# ThreadsPerChild: constant number of worker threads in each server process
# 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>

     I want to  know about  the difference  between worker MPM and
Prefork MPM , how to find out which one will be used by my apache
server and  the recommended one   for highly loaded server.If some one
provide me the link that best explains above two comparison also  be
very use full.


 Can any one guide me tuning to be  done for the maximum utilization
of the Resources  and better performance of the Servers.

 Regards,
lingu