This will consume around 250watts just for the card and will hash 650KH/s on most settings.
And if you have a low power cpu it will total around 320watts including the Hard Drive and Ram.
So if you're planning to mine ASAP, just first compute your power rates and also consider in looking for 2nd hand cards to lessen the cost. You can see here in the forums selling their mint 7950 for .5 BTC.
Hope this helps you.
P.S. $0.25/KwH is your breakeven point.
Where can you find them that cheap? I would be extremely happy seeing as how that's like 1/3 of retail value. You did mean 7950 and not 7850 right?