That's great news about board compatibility.
I checked the voltage inputs to the six-pin connector and the four-pin with two tails. Both tails are connected. I detected 12 and 5VDC readings. Both fans run on the R7950. The R7950 is indeed locked in to the PCI-E connector. Further, a manual "Scan for hardware changes" will not discover the R7950.
PSU is 550 Watts.
MSI 790X-GD70 main board with 1.H0 most recent BIOS
Chipset driver version, most recent: 8.881.0.0
VGA driver, ATI, most recent: 3.0.745.0
AMD 965 AM3 Black Edition processor