system crash

hi just bought 2 x 1gig memory sticks(DDR2 667 1GB X16) same brand and
speed. motherboard 775DUAL VISTA with INTEL CORE 2 E6420 2.1 GHZ. using DDR2
667when install both memory the system hangs after 3 minutes or so. but if
install one only (either) it works fine. unfortantely i loose DUAL CHANNEL.
the motherboard state's it can handle a max of 2 gig mem. why is this
happening and is there a way of using both without crashing. using vista
premium OS. if i use slower memory 750mb this system also runs fine. i am
certain it is the memory.
thanks.