RAM upgrade by adding 667fsb stick when limit is 400/553fsb?

What I understand, it is better to use the same exact type ,size and speed for all sticks, so whatever stick you have now, add same one, and it should be fine. but you should have rams that your mobo support, incompatible ram even if it ran, I think it can run with low performance. before I upgraded my motherboard, my ram wasn't compatible, the system used to beep twice indicating problem, and I had issue with performance and speed of the OS. I think it is due to this beside the motherboard being basic.