1 or 2 months ago, 1BTC=120-130$. Today, 1BTC= 900$ at Bitstamp rate. Is that means if I bought BTC 2 months ago and I try to sell them today, I earn 780$ per BTC?
The exchange rate has changed by almost 800 dollars. That means that if you had bought 1 bitcoin before the rise, you would have assumed the full risk of the price not rising (and possibly falling), and you would have been rewarded for taking this risk with a profit of almost 800 dollars. Note that you would be exposed to additional risks, including a possible bankruptcy of the bitcoin exchange you use, as nowadays even reiterated in mainstream media. See this CNBC piece on today's warning to this effect by the European Banking Authority for example.