Yes, the 280x is a 7970. The GHZ is just a name they put on it so that it seems special, but in truth it's the same card just clocked at 1000mhz or 1ghz.
So quick answer YES.
However the 280x may clock down to match that of the slower 7970, you can also overclock up the 7970 to 1000mhz though.
Also 750w is fine for 2 7970's in Crossfire.
-good luck