The 7970 is a relative efficient card (compared to the older models) and you've got quite a powerful PSU. I depends a bit on the power you PSU can actual deliver on the PCIe rails but I think it should be safe. The recommend minimal PSU for an HD7970 is 650W but as you can see in several reviews, the actual power consumption is significantly lower. These reviews recommend a PSU of 550W so it's not you're close to the lower-margin. In none of the reviews, the power consumption load (even overclocked) is higher than 400W. I guess your PSU is 80+ certified so that means you've got 560W available for components.