I recently got interested in BitCoin mining. Now I am considering buying 2 graphic cards for my current rig and let it mine whole summer. The problem is that I constantly read here how mining becomes unprofiting in the future (eg. you make less than the electricity bills). However when I use this calculator, I see that if I buy two 6950s I will make 500-550$ a month. This means that I will pay off the cards in 1,5 month(s) (this is with the electricity bills included) and everything I make after that is going to be a pure profit. Is there something wrong with my calculations?