if i were you i will at least get something that can double my current performance. crossfire is one easy way to get more performance but it will not guarantee double the performance since it depends on the scaling not to mention the scaling also different from one game to another.
before giving suggesting it is better to know what is your current system spec and if can your intended budget as well. we can recommend you any solution out there such as GTX580 in SLi but it will be useless recommendation if we can't know for sure if your rig can handle it without bottlenecking or you simply don't have such the budget