I'm thinking of purchasing an ASIC miner, but I'm not sure which one to go with as there seem to be multiple options. When they mention 20-60Gh/s, I'm not sure how many bitcoins can that mine in a specific time. I would like to know how to calculate how many BTC is mined with say 40 Gh/s.
ASICminer and Avalon are already deploying, and they will hit over 100Th/s before too long. Butterfly Labs has 75,000 chips on their way, each at 7.5Gh/s, this will account for another 560Th/s. Avalon has sold out batch 1 and 2. ASICminer is in the process of deploying their farm. BFL has 19500+ orders. Many of those orders are for multiple units.
Basically, many people have pre-ordered based on the idea that they are going to get rich quick, so they have overbought. This was part of why BFL has delayed so long I believe, to generate more orders than they would if people knew how far up the difficulty was going to go. Go ahead and order if you want, but realize that ROI could be much longer than you anticipate. Safe bet would be to wait a couple months to order, then you will have a much better idea. Honestly at this point, there are so many orders in front of you, it will only make a few days difference whether you order now or later.
By time you receive an ASIC that you order today, the orders ahead of you will have pushed the network to maybe 700Th/s. So your 40Gh/s is .04 Th/s. There are 3600 bitcoins generated each day. .04/700*3600=.2057 bitcoins per day. This will generate $6.17 per day, or $185 a month. Also consider that the network speed will continue to increase. So maybe after 6 months you are making half that.