This is to update to you guys that two HD7970 are in my machine and without external power. And it's silent! You only spend a fraction of money comparing to buying a new MacPRO, provided that performance is your most important decision factor. Meet the new King of Mac Dual X5690 3.46GHZ, dual HD7970, 128GB DDR3 1333mhz (8X16gb), two 500gb SAMSUNG EVO at 1.0TB raid0. The SAMSUNG SATA Raid 0: http://forums.macrumors.com/attachment.php?attachmentid=473007&stc=1&d=1400545178 The Raid 0 speed Test: http://forums.macrumors.com/attachment.php?attachmentid=473011&stc=1&d=1400545178 Cable used for SATA power connector: (note that you need to cut one of the edge for the connector.) http://forums.macrumors.com/attachment.php?attachmentid=473009&stc=1&d=1400545178 OK. The way I have done is using VBE7 (google the keyword) to lower both cards' voltage from default 1.17V to 0.956V, downclock only tiny bit to 925MHZ for GPU and 1300MHZ for memory. By doing this, HD7970 will consume less than 200W ( around 175W-200W). Second card in my mac Pro uses the remaining 2 SATA ports. Luxmark OPENCL test with both D700s (as it reported on my osx 10.9.1) with CPU+GPU, the default scene to be 5146. http://forums.macrumors.com/attachment.php?attachmentid=473023&stc=1&d=1400547538 ----Critical information about combination of the 6+8 power----Updated on May 19 2014-- It's critical and this is why I decide to post more instructions for you guys who wants to do it. This makes the theory profound and even lower risk for Mac Pro 5.1. I used HDsensor to get the reading. It also solved a myth that both 8+6 pins never draw the same currents. Experiments done with the lower clocked/volts HD7970 under Unigene Valley default demo. It reads 27W from PCIE + 28W from 6Pin + 70W from 8pin in total of 125W. (That's lower than what had been calculated). The readings show a necessary that power connecting from SATA port (calculated 55W typical capacity from SATA port) to be arranged to the 6pin on card, while power from the Mac Pro onboard boaster (75W typical capacity) need to be only connected to 8 pin. As shown here: http://forums.macrumors.com/attachment.php?attachmentid=473010&stc=1&d=1400546893 Thanks for Asgorath's post in this thread in #14, http://forums.macrumors.com/showthread.php?t=1514609 Enjoy your new KING OF MAC! There are bios posted here: http://forums.macrumors.com/showthread.php?t=1733650 Please don't be too serious about the maxed Xeon 3.46Ghz, these two D700 upgrade are far more important than the CPU itself. In other words, to be as competent as nMP, and for openCL which is strongly supported by APPLE in FCP and ADOBE's PREMIER PRO. Honestly, I tried single CPU x5670 with dual D700s, the BruceX test is the same 23-24 sec. Hope you guys can update yours if you have done it. Any questions for the instruction are welcomed.