That power supply really seems like it could run both cards on your system. The distribution over the rails looks ideal for a card like the 460. From the specification, it can put out 28A on 1 12V rail and 20 on the other. Those videos cards require around 13-14A and 160W each. That combined with your CPU requirements (around 6A) and other system components will probably put you around 450W at load. You have an SLI-ready motherboard?