kingjulian

Usually, the motherboard comes with four DDR2  DIMM sockets.  You mentioned that you had 1GB RAM PC2 4200U-444-12 (DDR 2 533 ).  You must ensure that there is one 1GB module or there are two modules of each 512KB.  If you have one 1GB module, get another 1GB module.  If you have two modules, get two others of the same kind.
DDR2  DIMM sockets always come in pair.  There are two channels (two DDR2 DIMM sockets for one channel).  You can easily see that the four DDR2 DIMM sockets are in two different colours.  Make sure that you insert the same kind of RAMs into the sockets of the same colour.  It is a good idea to have the same make and sizes of RAMs always.