I just bought a second HD 5770 and I wasn't sure if I needed to buy a new power supply or would my current power supply suffice. My setup is a i7 920 @ 3.8ghz, gigabyte x58a-ud3r, coolmaster hyper212 plus, corsair 650w, ocz 1600 gold(6gb) and an antec 900 case
The easiest way is...
to play a game let say crysis or something else a little heavy..then you check the power draw from the electrical line either with a power meter or with an ampere meter and do the calculation later.
Then the result of the measurement you can divide it with about 0.8
The is your power consumption although not 100% correct but it will give you a slight view of the power consumption.
You have plenty of available power, even with overclocking. At stock speeds, two 5770's only need about 16 amps. CPU needs about 10 amps. Overclocked, they will draw about 20% more. That still leaves you with about 20 amps for the rest of your system.
The usual symptoms of a gaming system with inadequate power is usually random reset/reboots under heavy 3D graphics load.