no vdrop, but i been problem getting my system stable,

right now i got my Q6600@3502, with 437fsb, i found out it was my ram causeing my stable problem, right now its at 505mhz but i dont get it, they used to be stable at 533mhz without overclock fsb,

Anyway my real question is if i go buy some 1200mhz ddr2 ram chips will i have this same problem and have to go underclock 600mhz? these are rated at 400mhz the ones i have