You would have to calculate the rate at which you purchase electricity at, then calculate how many bitcoins you can mine and how much they would be worth. on the Nvidia side, if you have any card worse than a GTX 460, I think you wont even break even. AMD cards tend to be better and you would need a card that is better than a 5670. Anything less than 60 megahashes isn't worth it these days, unless if you just want to get some sample money to gamble with.