That is listed as DDR3 1333.
I'd double check the ram that you have installed because I thought the MP came with 1066 MHz and I'm not sure that 1333 RAM is backwards compatible.
I´m pretty sure you need something off of this page (PC3-8500)
If you buy 4GB DIMMs you should definitely use registered. The next size up are 8GB and they are only available in registered spec. So you can mix them.
I have tried 1333 CL7 and they clock down to 1066 without problems.
My experience was with Hynix DIMMs which is the same as Apple's stock fit. I'm currently running mixed mode 4x2GB + 4x1GB with no measurable difference compared to 6x2GB.