I'm thinking of buying 4GB DDR2 1066 (2x2). But I've got 3 sticks of DDR2 667 in my system now. Can I leave two sticks of DDR2 667 in and place the 2 2GB 1066 in the extra slots? Would that be fine?
Yes, just make sure that they are running on different channels. Also check the voltage of both the new and old RAM and compare. You may not be able to run at efficient speeds if one requires more voltage.