So we have two scenarios:
A. I stop mining and keep 100 of mined bitcoins so far for the cost of one month.... and have them worth 200*X$ in a month
B. I continue mining and get to 160 bitcoins in 1 month for the cost of electricity and stuff for the cost of two months... and have them worth 320*X$ after that month
Hard to choose, I think I will go with B?
Question is:
Will you mine more Bitcoins until your hardware fails than you'd get if you sell your hardware and buy bitcoins instead?
If yes, mine along.
If no, sell hardware.
Currently nobody can give you an answer, as it depends on difficulty developments, that are difficult to predict. Here in europe for example I could buy 58xx cards in the dozens... and Bitcoin is barely known and unpopular, as they are heavily traded only in USD. Once MtGox also trades in EUR, I expect a HUGE increase of demand and miners!
It might however also be that now the ceiling has already been reached and we'll settle at ~1 million difficulty, only increasing when new GPU generations are released.
Just do a difficulty vs. exchange rate chart yourself and you'll see we're currently heavy in a mining zone, only held up by demand of people who heard "Bitcoin is the new sh!t" in the news. This can change any moment, a more realistical/sane exchange rate would be ~10 USD/BTC for the current difficulty. The longer we stay in this zone, the more people will order GPUs and bring them online, driving difficulty up.