What is the frequency of the existing Ram? Try to purchase another ram of exact same frequency of the existing one for better compatibility. However, currently it is very hard to find DDR2 ram any other than 800 MHz frequency. So if your existing ram is not of 800 MHz, get the 800 MHz ram although there might be some incompatibility issues. Normally if Rams of different frequencies are plugged they run on the frequency of the lower one.