Check your manual, or online support.
Some boards have pita requirements about what you put and where. You may have to match each slot w the same chip (2x256=512) or you might be able to drop 256 into the second slot giving you 320
Also depending on how old the pc is, you may just want to upgrade because adding ram onto 5400rmp ata 66 drives won't give you a noticeable improvement.
64megs of ram sucks, but make sure it is the weakest link first, or you won't be happy with the results.
JAson