thanks for the answer.
First i'm here only considering GPU mining.
I have this idea to build a MOBO with integrated RAM,CPU,DISK that can accept 4PCIe cards because i see here i can gain with the low power consumption ( as you said this components don't contribute to the hashrate) and hardware price.
I think the components that i have in mind will consume no more then 10W( all the RIG components except 4x GPU).
Research carefully how much CPU power is needed to run various GPU mining programs. Sometimes low-end Celerons can start to bottleneck GPU performance with certain mining software.