I don't think the 667 RAM is a factor in overall performance.
The performance gain by even a small overclock would greatly exceed the small gain you'd get switching to 1066 RAM.
Do you know what revision your GIGABYTE GA-MA785GM-US2H is?
You can use the free utility CPU-z
to ID the revision if you're not sure. Run the utility and look under the Mainboard tab. You should see the REV number out to the right of the Model number.