Based on information I found on the XG-P700 of:
DC: +3.3v +5v +12v +12v -12v +5.25
24A 15A 30A 22A 0.3A 2.5A
Which is 360watt rail #1 and 264watt rail #2. The 7970 requires 190 watts to operate. Your power supply will power the GPU based on your ratings, but you will need to make sure you are setting up the power cabling right. I find it hard to believe it has only one 6 pin power connector with 700watts. Are you sure it doesnt have an 8 and 6 pin connector? The 8 pin connector (not the 8 pin CPU power connector possibly) is just a 6 pin with 2 extra pins in it. It will fit the 6 win with 2 pins overhanging. I couldnt find cabling diagrams, so if you want confirmation, you can take some pictures and post them.
What I would do, order a better power supply, but if you are keen to use what you have, it SHOULD work if the power supply label is correct and accurate.
Short of another 6 pin connector, you will need to use a molex to 6 pin PCIE power cable adapter that lets you supply power from the molex power connectors. Like this one:
http://www.gttkc.com/shop/images/molex_pci-e.jpg