Yeah, adding more ram can lead to instability. (more than one slot) Take for instance the mushkin revision 2. Mushkin told me that by adding additonal sticks of ram it loses the ability to run as fast. (I am not exactly sure why, I'm sure some other person here could give you a long boring rant about it) I know that two chips won't run at 143 cas3 on my comp whereas either stick alone runs 152 cas2.
