Timeout 60
KeepAlive On
MaxKeepAliveRequests 150
KeepAliveTimeout 4
MinSpareServers 5
MaxSpareServers 20
StartServers 5
MaxClients 150

Your settings already seem to be well enough. However if you get too many connections to apache, you may consider raising "MaxClients" a bit to 200 or 250. You can also install "APC op-code cache" to speed up PHP page serving.

Thanks for all the help. Appreciated it!