As long as your RAM is the specification of Apple, it doesn't matter who manufactures it. Apple dont manufacture RAM
I still dont understand why only one core is being used.
Also it is best to configure the Mac Pro with 4 slots of RAM, to quote from Apple.
"The Mac Pro now uses faster 800MHz DDR2 fully buffered ECC memory, an industry-standard memory technology that allows for more memory capacity, greater bandwidth, and better reliability. To take full advantage of the 256-bit-wide memory architecture, four or more FB-DIMMs should be installed in the Mac Pro."
Alternatively you have some faulty RAM