I have a Striker Extreme motherboard and am using 2 x 1gb sticks of DDR2 Corsair XMS2. If I want to upgrade to 4gb, do I have to buy new DDR2 in the form of 2 x 2gb OR is it common practice to purchase another pair of the Corsair XMS2 and insert them into the remaining 2 slots? Obviously, the second option would be much cheaper.