Re: RAM confusuion

as most have said before it depends on your motherboard.
there are a few different ram technologies, all are incompatible with each other (you can get PC100, PC133, DDR 266. DDR333. DDR444, DDR2 and now DDR3 chips)
pc 100 and pc133 are obsolete, you cannot get them any more, if its ddr 266, 333 or 400 then they care compatible with each other, but they will run at the slowest speed (if you have a ddr266 and install a ddr400 to your machine then they will both run at ddr266 speed, the same is true for DDR2 and DDR3)

if you are unsure then simply take the ram module that you have with you to the pc store and ask for an identical one, they will be able to match it for you.