And why use 500-700w ATX for the motherboard, if an inexpensive, good quality power supply of 300-400 watts is enough, and connect the rest of the video cards to the server power supply?
Or does your server PSU not have enough power?
Because it only costs $10-15 more to get a 600w ATX supply than a 400w unit. That is a price of 15-20 watts/$ which is excellent.
I agree with you that modern computers use 500-700 watt power supplies and are easier to sell later.
But why use 2 power supplies? Is there not enough power in the server power supply or is there no adapter to power the motherboard?
This adapter costs less than a second power supply.