Zaffa said:since your running pc2100 id guess you have a rather old motherboard and there fore you are not running dual channel, if that is the case then pc3200 will work perfectly well with the pc2100 mem, but will only work at pc2100 speed
the other question is if your old motherboard supports double sided memory in all/some slots or not, if not, it could take away ½ your new memory so it will only work like a 256mb block even if it is a 512mb block, or it may not work at all