I would stay away from pre-orders as by the time you actually get it, the ROI could be who knows where. In my opinion, I would simply purchase bitcoins rather then mine them. I have done my fair share of mining and i'll tell you, the days of printing money with no effort are behind us. It is EXTREMELY competitive currently. The only way that mining will become more profitable would be if the exchange rate went up to 200 or so. At current prices, you can't get your money back easily.

That being said, heres the answer to your question based on TODAY's Difficulty.
If you picked up a 25gh/s and 50gh/s miner from BFL, that would put you around 75gh/s.

This would give you 1.01 btc per day approximately and come in under 4k but with shipping, etc... you'd probably land at 1btc per day. Like I said though, who knows when your miner will arrive and what the difficulty will be when it comes.

If you wanted to go with USB miners from ASICMiner, they are in stock, no pre-order and you have them at your door in 1 week. I ordered some last week on a sunday and had them on the next monday. Very quick and easy to set up


I used this calculator.
http://www.alloscomp.com/bitcoin/calculator