I built my computer around Xmas of last year, and it is working great. I initially had two sticks of this RAM in the third and fourth RAM slots (the mobo-manual said that this was the best configuration for this frequency of RAM). I set the timings in BIOS and they work just fine. No freezes, no shutdowns, games run good, etc.
My problem is I decided to get two more sticks of the same RAM because I got a great deal on them, but when I plug all 4 sticks into the mobo a memory checksum error occurs and Windows 7 will not boot. Is this a known problem with the ASRock 870 Extreme3 motherboard? I tried plugging in each stick one at a time, but encountered the same problem at a 6GB config.
So to recap, 2 2GB sticks in the white 3 and 4 slots works fine, but 2 more 2GB sticks in the blue 1 and 2 causes the computer to not boot.
With only the old memory installed, go into the BIOS and lower the memory clock by one step. If it's at 800/1600 now, set it to 667/1333. Save and exit and let it boot to the logon screen or fully into Windows then shut it down. Put the new memory in and attempt to boot.
It worked. Now I have 8 GB of ram operating at 733.4 MHz, so Leaps answer of stepping down the frequency did the trick. Tecmo's answer of upping the voltage may allow me to increase the frequency while maintaining stability, but I have not tried it yet.