Yes, you can crossfire them, just not 100% problem-free with that PSU.
HD 7970 tdp = approx. 250w at load
R9 280x tdp = approx. 250w at load as well
For both cards alone you will need approx. 550-600w raw power from psu during maximum load (got that figured from metro: ll)
Now, calculate the needs of a beefy cpu, overclock, hard drives, sound cards, case fans, dvd players etc.
You then realize that a good 850w 80+ certification PSU is a good option for you.
EDIT: you also want to make sure you have enough amps through the +12v psu rail. 30A i believe (for each gpu).
Take a look at this:
Power requirements for graphics card
gigabyte X58A-UD3R rev.2, core i7 975 3.9ghz (stock voltage), windows 8.1 64-bit, corsair 12gb DDR3 1600mhz 7-7-7-20 RAM, xfx HD 5970 + gigabyte HD 5970 (950mhz/ 1100mhz), coolermaster ultimate 900w PSU.
AMD Catalyst 15.4 beta
12:16 PM by