it is saying:
-total maximum memory is 2GB
-you have 3 memory slots
-each slot can hold a 256MB-2GB module of DDR SDRAM
-you can use either DDR200 or DDR266 memory only.
DDR200 is also known as PC1600
DDR266 is also known as PC2100
so your answer is NO you can only use a total maximum of 2GB in the computer. there is no reason for you to have more than 512MB as it will not be used by your computer except in rare cases.
almost all games and applications will never use more than 512MB. running a large server or a rendering slave are 2 rare exceptions.