Sorry for the late reply, was out of town. If you have the 2gb version and can find another 2gb version to CF then it would work out pretty well. But, before you go for it, I would check to see what games support crossfire that you like to play. I know most big name games do support it but indie games don't always.
Also I made a mistake earlier and thought you had a 650w psu not a 600w psu. I would definately get a new PSU for crossfire since I would not trust a 600w power supply to handle the needs of 2 cards.