1) What is your break-even point for power cost at BTC price and difficulty level?
2) How long, under what assumptions, until ROI is achieved?
IMO, the game is now over for GPU mining as it was for CPU mining before.
Unless your calculations show you mining at a profit with that hardware at difficulty level well past 60 million in two or three months, currently around 4 million, then make the sale, pocket the cash and consider you had fun while it lasted.
It is a sucker's bet to be buying new GPU hardware just for mining at this point.
Continuing to use existing GPU's, you might do OK for another couple of months. At current BTC exchange levels.