I am interested how you guys can be so sure that the PSU will supply enough power. As true multi rail PSU's are very, very expensive (used in laboratories etc), and due to safety cut-out (overvoltage) requirements in domestic appliances, the quoted PSU is not a "true" dual rail PSU (most PC PSU's are not - rather they share a single rail with multiple parralell outputs). Under ATX standards, 12V+1 is dedicated to the CPU leaving other rails (in this case only 12V+2 (15A)) for the rest of your system (including the GPU). That PSU will not power your system, and to be honest i am suprised it is coping with the current set up. If you want more explanation, read the multi/single psu link in my sig.
You will need to get a new quality PSU such as the TX750W from corsair.