What is the fastest RAM for GA-EP45-UD3P with 8GB

Hello,
I've been having the most frustrating time with a GA-EP45-UD3P(rev 1.1) and RAM when I fill all four banks with 2GB modules (8GB total). I've tried Corsair 8500 and G. Skill 8500. I've spoken to Gigabyte tech support
for a total of about 12 hours now during the past 2 months. For reason when all banks are filled with 2GB sticks, no matter what values in the Bios I change/tweak, the modules run at PC6400(800).

If I wanted 800 speeds, I would have purchased PC6400 modules. Apparently, if all banks are filled, the RAM speed needs, and the system does it automatically to be underclocked.

I'm wondering if anyone has had success in running 8GB of PC8500 at stock speed. And if the reality is that in order to use all four banks the RAM needs to be underclocked, is it recommended to get much faster RAM so at least I'll be able to run it at a speed higher than 800? If so, does anybody have recommendations?

Any comments, input, direction to other resources, advice, etc. will be EXTREMELY appreciated.

I have the latest BIOS already.

Check with CPU-Z that it worked and run memtest86+ and check for errors.