u3: 1watt/GH
7970's: 200-300watts for 0.6-0.7GH/s
And whoever is paying for electricity will notice a jump of that nature...
Each card would only do 700-740kh on scrypt mining.
Best bet is get a gaming computer for gaming, and a Bitcoin miner for mining.
Or he could mine X11 which would give a lot more..
Other algorithms does exist...
I would not recommend mining with a Gaming PC since the cards will take damage.
Very true, I don't know how profitable those are tho. Supposedly I've heard news about x11 or x13 miners at the end of 2015.... Forgot the source at the moment
Edit: Found source https://www.cryptocoinsnews.com/asic-coming-x-algorithms-darkcoin-watch/