Cooler Master G Modular PSUs are mediocre quality (Tier three) PSUs.
PSU Tier list:
http://www.tomshardware.co.uk/forum/id-2547993/psu-tier-list.html
Since HD7970 can consume up to 250W, you'd do well with 650W PSU.
Aim for a Tier two PSU, preferably Tier one PSU if you can afford it.
For Tier two PSU, i suggest Seasonic G-650,
pcpp:
http://pcpartpicker.com/product/R7V48d/seasonic-power-supply-ssr650rm
And for Tier one PSU, i suggest either Seasonic X-650, Platinum 660 or PRIME 650,
pcpp: http://pcpartpicker.com/products/compare/Fgwqqs,CfXfrH,nn648d/
Note: Seasonic PRIME 650 is the best 650W PSU money can buy at current date. (My Skylake build is powered by PRIME 650, full specs in my sig.)
PSU review...