The performance of two 512 mb sticks should be higher in dual channel mode (128 bit interface 64x2), than 1 gb stick in single channel mode (64 bit interface). The difference in higher latency in dual channel mode wouldn't be enough to make it perform below single channel mode. If I were in your position, and if it could be afforded, go with two sticks of 1 gb memory. There's nothing wrong with purchasing value ram, just make sure that it'll work with your motherboard.