The profitability calculator assumes difficulty stays the same for the entire time period, so be careful getting too excited about a short return on investment timeframe. When it comes to Bitcoin it is safest to assume the worst. I personally multiply the difficulty by 10 or 100, even 1000, so I get a worst-case scenario. Remember this is investing, anything you put into Bitcoin should be assumed to be lost funds.
I would also like to touch on mining like you mentioned. A lot of newcomers get caught up in mining and want to buy a lot of expensive mining equipment. This is generally the worst way to profit from Bitcoin and its altcoin counterparts. Bitcoin is designed to make mining marginally profitable given a hash rate that most people have. Most people have your hash rate, and thus it is marginally profitable to mine. I'm not saying that mining is bad, I'm saying that if you really want to make money investing, I would suggest buying the coin directly, rather than mining it.
The 50 GH/s mining rigs you mentioned are known as ASICs (Application-Specific Integrated Computers). They are extremely efficient and fast at mining, however they can only mine, nothing else can be done with them. The ad you saw for an ASIC miner was more than likely Butterfly Labs, commonly just called BFL. They started taking ASIC orders in June 2012 and have still not delivered any ASICs to actual customers (they have supposedly shipped ASICs to media outlets). I personally would never purchase an ASIC from BFL. The only company that has ever shipped ASICs is called Avalon. Avalon only takes orders in batches and currently you can not order from Avalon. ASICs seem like a good deal, but the problem isn't buying them, it is physically receiving the item as described.
The Bitcoin Wiki can give you a lot of information about Bitcoin itself (
https://en.bitcoin.it/wiki/Main_Page) I suggest reading as much about the coin itself as possible, it takes some time to fully understand just what Bitcoin is, which is unfortunately a massive barrier to its adoption.