Hello,
See page 43 of your manual. I believe that they are calling it "Graphics Core" with a a couple of sub-headings below that activate once you come out of Auto voltage setting.
PCH isn't tied to the vcore or PLL. PCH controls I/O ports and slots. These includes Graphic cards and very likely your SSD. People with many HDD's or using multiple video cards might need to set PCH Voltage at 1.20v if they overclock to high multipliers and are using SLI/Crossfire, to avoid crashes in the video driver, just like raising VTT is necessary for 4 sticks of RAM and high ram clocks.
Compare your Graphics Core voltage in BIOS against the GPU voltage of your 6850 Video card. Then raise Graphics Core settings in incriments to 1,15V in BIOS to see what it does to your SSD and 6850. Make sure that you are using the latest BIOS for your motherboard.
Regards,