Get an R9 270 or 270x, both are more powerful than the 660 and available for around the same price. Also consider getting a second hand 7870/7950. Crossfire/SLI is only good if you have high-end gpus and there's no single gpu upgrade path, so you whack another one on. Scaling on multi-card configurations is also a problem. Are you sure your motherboard supports crossfire?