Mushkin wasn't in the sub-category list :S
I just bought 2x2GB Mushkin RAM and they are 1066MHZ.
However, when I run CPU-Z it says that they are running ar 400 MHZ.
Maybe I'm reading this wrong but I'll post a picture.
Your ram is running at 800, not 400. Cpuz gives you half the actual speed. You can check your board manual, and try increasing the speed to 533 (or 1066, depending on your board readings) in the bios, or overclock the cpu if your board permits it. But most oem boards by dell, hp, etc. don't allow it.