You don't really need a calculator 
The advertised TDP of a 6870 is 150 watts. That's 3.6 kWh per day if you run it 24/7. Multiply that with your effective cost of electricity per kWh and you've got an estimated daily additional electricity cost.
Example: If you pay 10 cents per kWh, your daily cost is approximately 3.6 * 0.10$ = $0.36/day.
The advertised TDP of a 6870 is 150 watts. That's 3.6 kWh per day if you run it 24/7. Multiply that with your effective cost of electricity per kWh and you've got an estimated daily additional electricity cost.
Example: If you pay 10 cents per kWh, your daily cost is approximately 3.6 * 0.10$ = $0.36/day.
Excellent, thanks for your input
That has cleared some things up for me, my understanding of electrical units is...shoddy...
I've worked out investing £300..$470 odd i think..in a 7970 should double my return (and double electricity output :S) which in the long run should work out great. Assuming difficulty mining ltc is managable.
Can anyone shed some light on mining difficulty? In their opinion will it become very difficult to mine on a bog standard rig such as my hypothetical one in the near future?