I'm planning to get a 1440p monitor and i'd like to get a second video card to make up for the performance loss. But my psu is an antec hcg 620m and i'm not sure if it's enough. My current asus 670 is oc to 1125/6500 and i'd probably go for a similar oc on the second if possible. My cpu is an i5-3570k which i will oc later when it starts to get slow.
More than likely you are fine. I know my 3570k with a single 670 pulls about 310 watts from the wall peak, which puts it at about 265 delivered to the machine. Even if you add a solid 200 extra for the second card you are only at 465 (likely) - 500 (max) or so and your PSU is rated for up to 575 so that even leaves you a little head room for age and things.