You probably have a kit of 2x256MB
DDR2-100 does not exist.
Older Ram would be DDR266, DDR333 or DDR400
It is common on those bars to have kits of 2x256 to have 512MB of dual channel.
Common Ram would be DDR2-533, DDR2-667, DDR2-800, DDR2-1066
Common bars are 512, 1024, 2048... I have not seen a 256MB of DDR2... but its possible.
So in order for anyone to help, research what you have.
Does it not work or does it not fit would be my question as you say DDR2 yet its 256