Hi billy - 700w would be enough. Are you limited to $65-75 for a
PSU. A good rule of thumb is you should spend approx 10%
of your system on a PSU. Your PSU slection tends to look
at your system as unbalanced from that perspective.
For a system using a single Radeon HD 7970 GHz Edition graphics card AMD specifies a minimum of a 500 Watt or greater power supply. The power supply should also have a 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.
Total Power Supply Wattage is NOT the crucial factor in power supply selection!!! Total Combined Continuous Power/Current Available on the +12V Rail(s) rated at 45°C - 50°C ambient temperature, is the most critical factor.
You may find power supplies on the market that supply more than enough Wattage to run the system. However, many of them lack the 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 overstate the power supply wattage, usually by at least 50 Watts, to take into account those power supplies that have the weaker +12 Volt rail(s).
The OCZ ModXStream Pro Series 700 Watt (OCZ700MXSP), with its combined +12 Volt continuous current rating of 46 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. This power supply is a poor choice because it is unable to deliver 700 Watts.
Since you will be using about 400w under load at stock settings and have a k series processor I suspect you will overclock. Though you are still in the safe range if you might want to upgrade to a better supply around the same wattage. If you plan to crossfire down the road then I would recommend something around 800w.