i have an older system here that i'm updating and reformatting. it currently has 2x128mb of pc800-45ns rambus memory. i want to add on another 2x128 or 2x256 but don't know if other faster speed ratings would be compatible, like pc800-40ns? or can i run ECC memory at non-ECC? also, what's the deal with 144mb and 288mb sticks??