If you guys are seeing 4GB ram clocking issues, especially with 2x2GB modules the issue is TRFC.
Asus in their wisdom have limited TRFC to a max of 42 unless your modules have SPD higher. I have an SPD for the OCZ 8000 ReaperX which gets round this issue , if you need it come post over on the support forum.

I have a feeling those of you running 4GB 4x1GB modules may need to set higher than 42 also, it makes a huge difference to stability.

Just a heads up, Asus know and we are beating on them to fix this. It effects all 965,P35,X38 and X48 boards