I have a computer with 256 MB PC2100 RAM (2 dimms of 128mb) on a MoBo that only has two slots. I want to upgrade and was thinking of buying one stick of 512MB and ditching one stick of the 128MB bringing the total amount of ram up to 640MB. Would this be ok or is it not good to mix two sizes. If it is ok to mix them - Which slot should the 512 go into and which should the 128 go in to. i have a compaq Pressario 7000 - if that helps -- it is a compaq mobo i think -- if that helps to answer the question any.