You are always going to get better performance with two cards vs one , so if you have two now keep them unless you plan to get two 570's in sli. You should be able to do quite well with what you have but there is always the unknown and a recent article in TomsHardware shows exactly that.
http://www.tomshardware.com/reviews/battlefield-3-graphics-performance,3063.html
There were some unexpected results in this games benchmarks with various cards. Some people have said that I am crazy or don't know what I am doing when I say that in my computer I have three GTX 580 cards. The reason is because of situations like what is written in this article. Now granted not everyone can afford to buy three 580's but I'm not saying that I can easily do...