Considering the fact that 2x4850's will require about 500-550W and you have a Corsair 650, you're good to go with a second 4850.
I would opt for this over a single 5770. A single 4850 is almost as good as a 5770 except the DirectX11 feature. Now, get two 4850's and there's your answer! Both cards have the same No. of streams processors. The 4850 is 256-bit and the 5770 is 128-bit. But the 4850 is DDR3 memory while the 5770 is DDR5.
Considering the above specs, dual 4850's will be the better option. Plus, you have the power to work those bad boys!