Higher in synthetics? Yea. Higher in games? Maybe. Best choice? No.
Id go with a single 4870. This will eliminate Crossfire problems such as lack of support/compatability. And crossfire wont necessarily let you crank games up higher, they will just run smoother with possible higher FPS. Youre still going to be limited to to 512mb, and if crossfire isnt supported period (which is often isnt) youre going to be left with a single average card. Which is why I susgest a single 4870 with 1GB, such as the Sapphire model under $200 http://www.newegg.com/Product/Product.aspx?Item=N82E16814102801
I just droped crossfired 4850's for a single GTX 285, and im telling you that it was worth it.