Which is the best memory compared to performance?

I have been told that I can’t fill all the four slot of my motherboard with DDR2 1066MHz. can I fill all four slots with ram DDR2 800MHz or get another set of DDR2 1066MHz are the possibilities . But one of my two sticks is not passing the diagnostic test. I am confused which to use. I would like to fill my motherboard potential to 250. Do you know any memory that has great performance, need help? The system is of
CPU: Black edition Phenom II Deneb 965 Quadcore 3.4 w/Corsair H50 liquid coole.
OS: Windows 7 64bit home basic.
GPU: Nvidia geforce fermi 480 sc.
Ram:Crucial x2 2gb sticks 1066MHZ (4 gigs).
Case:Coolermaster HAF 932 fullsize.

You can get two sticks of DDR2 1066MHz or you can get DDR2 800MHz. but I think that the 4GB is more than enough if you are thinking of getting 2* 2GB kit. While 8GB will overkill for many of applications except the heavy video editing application.

While I think that the memory DDR2 1066MHz will give you the fast performance. The system will be working really great. These memory have been developed for the higher demand as now everything is just speed I would like to tell you go with DDR2 1066MHz. As I use the DDR2 1066 MHz it is the fastest I can tell.