So I decided to upgrade my Core 2 Quad build to 8 GB RAM. Here is the specs:

• GA-P35-DS3L
• Core 2 Quad Q9550
• Radeon HD7850
• SB Audigy
• 2x 2GB DDR2 800 MHz Kingston
• 2x 2GB DDR2 800 MHz GOODRAM
• some HDDs

My problem is that the Kingstons and the Goodrams refuse to work together at 800 MHz. When I set 800 MHz for RAM, the system turns off and on by itself several times, after which the memory clock goes down to 667 MHz.
The Kingstons alone or Goodrams alone work fine at 800 MHz. So my understanding is that they are simply incompatible with each other at 800 MHz clock.
The system works stable at 667 MHz. Should I just leave it as is? Maybe it's possible to set all the CL values manually to get it to run at 800 MHz, but I don't know much about it. Buying more compatible RAM is also an option, but only if it's really worth it.
Is there any significant performance gain in 800 vs 667 MHz RAM for Q9550?