That 550w is probably just fine for a single 4870. You aren't OCing your CPU, and if you stay away from that with your video I would feel comfortable with it. Once you start to overclock things it starts to draw more power, and while you meet the recommended requrements for a 4870, that is based on stock specs. Dual 16A rails should be fine.
A corsair 750 is complete overkill. You would need it if you were going to run dual 4870's though.