if ur mobo isnt dual channel then u can.
if its like mine, where i have 4 slots, must fill 2 slots at a
time, each with exactly the same memory, (e.g. 2X512MB and 2X1GB to
make total of 3gb ram)
if u dun have dual channel, u can insert for example 2gb stick +
512mb stick to make 2.5gb ram