You look at the total amps of the first card and the total system requirements. You add additional total wattage of a few 100 for the additional card. However, the GeForce basically just runs on the PCIe power, so it doesn't take much at all.
If you where running two GTX 260's, I would recommend going with a minimum of a 650W power supply. Check the PSU sticky for more links and info...
The GeForce GTX 260 will draw 2.5 Amps at idle from the +12 Volt rail
The GeForce 210 will draw 0.5 Amps at idle from the +12 Volt rail
Together they will draw anywhere from 3.0 Amps at idle to 17.74 Amps under full GPU load from the +12 Volt rail.
If you were to overclock the graphics cards the current drawn will increase since they will consume more power.
Your Corsair CMPSU-550VX will provide up to 41 Amps on the +12 Volt rail only when needed. There are other system components, other than the graphics cards, that will also draw current from the +12 Volt rail.