Be aware of doing preorders. BFL has a history of not delivering on when promised. The uncertainty of getting mining equipment in future time frame adds more risk to the economics of the mining operation. I paid for a pre-ordered Jalapeno rig from BFL, by the time I got it up and running, I estimate about 2-3 years for that original investment to pay off, or maybe never as the network hashrate rises exponentially. It's better to stick to equipment to be delivered in a short time frame, and buy something that can be paid off in 2-3 months of running it based on the current network hashrate.
Edit: At the point I committed to preorder, I calculated a 4 month payoff at the network hashrate at the time.