i use 5 of these to mine on multipool and granted they do not bring in alot but i do have fun with them ... there all overclocked also
Let's say you are close to 40 GH / s. How much would be the minimum to make it profitable?
That would depend on how much you paid for the miners to begin with. Then you'd have to know how much you would earn per day and then divide the cost of the miners plus the cost of the electricity to run the miners (if you even care about that) by the earnings per day. That will give you the number of days until you break even. After that point it's all profit.
Right now it looks like you would earn about 0.01 USD per day assuming 40 GH/s, 10 watts power usage, and 0.12 USD/Watt of electricity with the current price and difficulty of Bitcoin.
Here is a link to the site I used to get that info: