One of my systems has an OC'd Q9550, 4 GB RAM, a GTX260 - a card with close to identical power requirements to a GTX460, a Gigabyte EP45-UD3P motherboard, 3 hard drives and an optical, and a Soundblaster card all powered by a Corsair 750TX.
Running 3 instances of Prime95 to load 3 cores of the CPU and 3DMark06 to load the fourth core and the GPU, it pulls 375 watts from the wall as measured by my Kill-a-Watt meter. Figuring 80% efficiency, the system pulls 300 watts from the PSU.
A second Gtx460 would pull another 150 watts or so for a total of around 450 watts. Although I would prefer a better power supply, your PSU is adequate in terms of overall power, 12 volt capacity, and total number of PCI power connectors.
Your first sign of inadequate power would probably seemingly random reset/reboot cycles during heavy (gaming) 3D loads.