The first factor you have to consider while shopping online for 2GB DDR2 RAM is the intended usage. For instance, a RAM designed for MAC will not work with a Windows PC and vice versa. Make sure that the model you have selected is compatible with your laptop, desktop, MAC or server. The next factor you have to consider is the compatibility with your motherboard. Ensure that the model you choose has a frequency that matches that of the motherboard. Mismatch in frequencies can lead to damage to vital components. Once you have found the right model, you can complete your purchase at the retailer who offers you the best deal.