I found a solution so as to work 2x2gb (800mhz). I get the same behavior by my macbook.
" Hi all!
I want to report my experience of today:
I have a 15" macbook pro (MB134LL/A) 2,5 Ghz core 2 duo.
I also have an hp 6930p elitebook that I don't use (I received it for work but I use the macbook pro): this laptop come with 4 gb ram (2x2gb, PC2-6400, 800 Mhz).
My macbook came with 2x1 gb ram (PC2-5300, 667 Mhz).
So I wanted to change my 2x1gb with 2x2gb: my first test was to change only one slot so I installed 1x1gb (the original) in bank 0 and 1x2gb (the hp one) in bank 1.
The macbook booted correctly and system profile showed 3 gb of installed ram: of course the 1x2gb was downclocked from 800 Mhz to 667 Mhz.
Second test was to replace both the slots with the PC2-6400 800 Mhz: the system didn't boot, I could hear the superdrive, but the screen was black.
I tried to boot the system with only 1x2gb 800 Mhz but the results were the same.
So, searching the internet I found my solution: SPDTool (to modify the SPD EEPROM).
I install 1x1gb (original) in bank 0, 1x2gb (800 Mhz) in bank 1 and booted windows thorugh bootcamp.
With SPDTool I read the memory in bank 1, changed the CAS from 2.5 ns (800 Mhz) to 3 ns (667 Mhz), Fixed the checksum and wrote the new data.
Then I installed the modified 1x2gb in bank 0 and the other 1x2gb (800 Mhz) in bank 1 and the system started correctly with 4 gb of ram.
Hope this helps somebody smile.gif "