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.
I tried searching in Auctions Section. But I am unable to find any second hand 7950 cards.
