1024 is the magic computer number. when going from byte, to kill byte, to mega byte, to gigabyte, the factor is 1024, not 1000.
depending if your motherboard supports dual channel, it might be a better idea to get two sticks of 512mb ram. if it doesn't support daul channel, then its better to get just the one stick. it also depends on the type of ram you already have on your mobo (if you're upgrading) and depending on the number of slots you have.