Yes you can crossfire with the 4670 but the second card needs to be a 46xx series, as you have it not there is no possible way to crossfire them, sorry. You will want to return the second card. I would suggest upgrading to a better card such as the 6870 which is more powerful than two 4670's combined, that can also run dual screens.
Your current setup can run on even 430w of power.http://www.newegg.com/Product/Product.aspx?Item=N82E16817139026
But if you upgrade to the 6870 (best option) then you should stay with the new 500w model.