Most systems I think run better with pairs but theres nothing to stop you just running with one stick, when you use two it spreads the workload from you computer to help it run faster, where are you from? If you from the UK then go to Crucial like Muz mentioned then use there scan to find out what type you need then go to Ebuyer and see how much the same type is on there as they have very good ram offers. I would go for two 512 and take it to a gig if you can afford it.