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.