I agree they are so close that you might not be able to see any differences. Like said above one card vs two cards. One card uses less power and generates less heat. Here is a bench mark.
I couldn't find the 4850 so I used a 4870 vs 460 and you can see that one on one the 460 is better than the 4870. You put two 4850's then you might come out very close to each other.
The 460 is the best way to go. The second link is a 3870 vs the 460 so put the 4850 in the middle and you will have your answer and even with two you still would not over come the 460 and good luck to you.
4870 vs 460http://www.anandtech.com/bench/Product/304?vs=313
3870 vs 460http://www.anandtech.com/bench/Product/542?vs=514