What is the fastest DDR2 RAM I can run in my I530?

I recently rebuilt my I530 which I bought brand new. It was originally equipped with 32 bit Vista that has long gone the way of the Dodo. It has over the years gone to 64 bit 7, and then 10. It has been a trusty, and reliable unit with various hardware changes to improve performance or keep it updatable. A couple of months ago it started acting up, and I found a number of capacitors had failed. What to do?

Enter Ebay. I found a new DG33M01 board for 50.00 shipped that would accept a free Intel Q9650. I assembled all with a new 650W full modular PSU, new case fan, Cooler Master Intel Extreme CPU cooler, ASUS GTX 770, Crucial MX500, 500GB SSD, and DDR2 800MHZ RAM. All said, and done it cost me about 2 bills to refurbish this unit, and it operates like new with no issues. I use this for personal computing, and playing HALO MCC which I play at 1650 x 1080 with everything turned up, and no issues.  Based on research I think I've reached the pinnacle of this board. That being said I received 8GB of Kingston HyperX 1066 ram for free. I installed it just to see if there would be any difference but I noted in the BIOS that it shows 800MHZ instead of 1066.

A CPUID, and Passmark RAMMon check shows all four sticks as Kingston DDR2 6400 800MHZ. What I'm trying to find out is if the motherboard is simply restricting the speed, and installing a default value or if there is an issue with the RAM? Possibly mislabeled?  It sure looks like DDR2 HyperX, and It works fine but... It seems to me that if the board wasn't capable of running 1066 MHZ it would either not boot or throttle back but it would at least show the correct speed rating. Is this board even capable of running 1066 RAM or is 800 the max? If so what is the controlling factor?

Thanks for any input, and enlightenment.