Well People say that most pc100 memory can run at 133 with no problem, I don't know about that, but if you don't want to risk ruining your memory. You can run your bus at 100, and put both in, which should work. If you want to be more risky you can run it at 133 and try to see if both works. In W2k 256 is useful and 128 is minimal. Another tradeoff, is that if you have to run your bus at 100 you see you are losing 33 percent speed on it. I would start with the least risky first, if you don't realize perfomance problems, stick with it. Then if you really want to feel the need of the power try the riskier situation. If you have to go with just one, the 256 is the better deal