I received 2-2GB sticks of DDR2-800 Crucial memory (CT25664AA800). After installing them on my motherboard next to my old RAM, I couldn't get Windows, or any CD's to boot (Windows installation disk and Ubuntu live cd). I could eventually get into the BIOS, and the system beeps reported that everything was okay, but whenever I tried to load an operating system, I either got an error or my system restarted. My motherboard is an Asus P5Q-SE2. I currently have 2-2GB sticks of DDR2-800 RAM (I forget the manufacturer), which I know has previously worked with extra sticks of RAM before. One time I added two 1GB sticks of DDR2-667 RAM with no problem. I know that the sticks are in the correct position. When the new sticks are run by themselves, there are absolutely no problems. My suspicion is that I would need to change some BIOS settings. Any help would be greatly appreciated!
You didn't have to before because you used slower 667 chips which forced the speed down which lowers the voltage requirement.
If your trying to use both sets you must know what voltage each wants to run at 800mhz. If one set wants 1.8v and the other wants 2.0 you can try 1.9 as an example. You can also just set the clock to 667 and they will probobly all work together at a lower voltage at the lower clock.
As for Isn't that dangerous, why do you think the setting is there?
Again you need to get the complete specifications for both sets of memory. Write them down, you should find information in the following form,
Ex. 800mhz 5-5-5-18 2.0v 667mhz 5-5-5-12 1.8v
Don't just start raising the voltage! as you can see there are other issues.
Find out the information and post it here if you want some suggestions to try.