well, if it tells you you need to buy pairs then yes you will have to buy 2 sticks .
how many slots do you have ?
you could buy another 2 256 sticks so that you will have 1gig altogether, or you vould buy 2 512 sticks and have 1gb + 512 from the previous two sticks.
if i were you id do a little more research on the
mobo you have and of the different configurations of
ram you can put on.
P.S sometimes the User Guide for your motherboard has instructions about
RAM, so go ahead and read it