600W is plenty for pretty much any single GPU setup. You should be fine.
If you haven't bought it though, I'd go for a better PSU. It's solid enough but you can probably do better for the price in terms of quality and usable wattage (it's much closer to a 500W PSU than a 600W in real terms).
For a system using a single Radeon HD 7970 GHz Edition graphics card AMD specifies a minimum of a 500 Watt or greater system power supply. The power supply should also have a maximum combined +12 Volt continuous current rating of 42 Amps or greater and have at least one 6-pin and one 8-pin PCI Express supplementary power connectors. The combined +12 Volt continuous current rating of 42 Amps is the minimum recommended if you want your system to be able to run GPGPU processing or GPU stress testing utilities.
Total Power Supply Wattage is NOT the crucial factor in power supply selection!!! Sufficient Total Combined Continuous Power/Current Available on the +12V Rail(s) rated at 45°C - 50°C ambient temperature, is the most critical factor.
Overclocking of the CPU and/or GPU(s) will require an additional increase to the maximum combined +12 Volt continuous current ratings, recommended above, to meet the increase in power required for the overclock. The additional amount required will depend on the magnitude of the overclock being attempted.
You may find power supplies on the market that supply more than enough Wattage to run the system. However, some of them lack Sufficient Amperage capacity on the critical +12 Volt rail, which is necessary to properly power the critical components in the system (i.e. CPU and GPUs). This is the reason why graphics card manufacturers may overstate the power supply wattage, usually by at least 50 Watts, in an attempt to take into account some of those power supplies that have the weaker +12 Volt rail(s).
The OCZ ModXStream Pro Series 600W (OCZ600MXSP) power supply unit, with its maximum combined +12 Volt continuous current rating of 42 Amps and with one 6-pin and one (6+2)-pin PCI Express supplementary power connectors, is sufficient to power your system configuration with a single Radeon HD 7970 GHz Edition graphics card. Any attempt to draw more than 600.4 Watts from this power supply will cause it to shut down.
I would recommend this Seasonic M12II-650 (SS-650AM) for $99.99 ($84.99 after $15.00 rebate) before I would ever recommend the OCZ ModXStream Pro 600W (OCZ600MXSP). At least the Seasonic M12II-650, with its maximum combined +12 Volt continuous current rating of 53 Amps would provide you with some spare capacity to handle overclocking of the CPU and/or GPU.