You can download a program called Aida32 which shows exactly what type of memory you have. You need the same type, so that its compatable with your motherboard (you can also check the manafacturers website for your motherboard), once you have the type of memory the next thing is size, 2gb is usually enough for most applications and speed you will want all memory sticks the same speed (eg. if its 2 x 1gb sticks) dont worry if the memory is faster than your motherboard (it will slow down to match) but dont buy anything slower.