Which FPGA should I buy, and where ?
FPGAs purchased today are no longer viable mining hardware, after considering the capacity coming online from ASICs. ASICs are just that much more cost efficient to procure and operate.
As far as calculating profit, that will depend on future difficulty and future exchange rate, neither of which is known yet.
If you have the specs for an FPGA you can calculate how much it will earn (gross revenue) at the current difficulty:
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http://tpbitcalc.appspot.com/ That is slightly incorrect.......
You should state FPGA's bit-mining kit from the major suppliers in not currently cost effective, the actual FPGA's can be had very cheaply on Ebay. and are cost effective when incorporated into a home brew system...