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.