Well, the basic power Antec model isn't superb, but those cards are pretty efficient so I do believe that your PC could handle it. It would not make ME very comfortable, as I like to have my macchine operating in the sweet spot of efficiency of 40-60% capacity. Also you do not mention overclocking, which is also why I choose to have quite a bit of headroom.
If it were me and I wanted to get the most performance out of my computer, including overclocking and running the top software titles maxed out, I would invest in a better power supply of at least 650Watts. I use a 750W PSU with a single HD 6950, but I overclock and have a heavily modded machine with many fans , lights, etc.
You could however go ahead and order the second card and save towards a PSU, but i've given you my best recommendation.