Ok, so I got a reply from GGTS and this is what they had to say:
Thank you for your kindly mail and inquiry. Unfortunately, due to hardware limitation, GA-MA790X-DS4(rev. 1.x) does not support 6 core CPU because it's DDR2 slot mainboard. It can't be changed by BIOS update either.
It does not support 6 core CPU because it is DDR2 motherboard? That sounds a bit iffy to me. My brothers motherboard GA-MA790X-UD4P is also a DDR2 exclusive motherboard and it supports the CPU. Does anybody have anything to add? Somehow I am not buying this DDR2 slot motherboard explanation. I have however asked the tech to elaborate for me, but he/she has not replied yet.