think I've answered my question myself. recommended psu for both graphics cards on their own is 750w so its a fair bet that I'll need a more powerful PSU to get this to work - ah well. Feel free to delete this post, I can't see how I can do that myself
For a system with a single GeForce GTX 590 graphics card NVIDIA recommends minimum of a 700 Watt or greater power supply with a +12 Volt continuous current rating of 50 Amps or greater and with at least two 8-pin PCI Express supplementary power connectors.
The Quadro FX 4600 has a maximum power consumption of 134 Watts (i.e. may draw up to 11.2 Amps from the +12 Volt rail).
The 850 Watt Corsair power supply unit with its +12 Volt continuous current rating of 70 Amps will have no problem powering your system with both graphics cards under full GPU load and the overclocked CPU under 100% load.
If your system configuration were to draw more than 70 Amps (it's usually set slightly higher) the power supply's over current protection circuit should shut down the power supply.