on the gigabyte K8N it will disable ram in certain instances.
although i don't see an issue with your 512 meg ram and the board, there may be an issue between the 256's and the 512.
if i am correct, you have to use either 1, 2 or 4 of the slots, not 3.
what i suggest here, is buy another identical 512.
then you may even manage to make it run dual channel.