Are there any FPGA boards that one can actually purchase for less than one arm and one leg?

Just wanted to put in my 0.02/16.9 bitcents of wisdom. If you go for the ATX power supply, you could put a connector for it on the board, with the proper "hotwiring" of the green lead etc. so people don't have to mod/vandalize their power supplies.

http://reprap.org/wiki/File:Cache-3408779902_7575952594.jpg

or maybe it is way to early for this kind of practical considerations... :-)

Why not get rid of the x86 board and arduino, and just get a embedded system that will cover it, has lower power consumption, and all that great stuff? Some come with OSes on it. Xilinx has FPGAs with embedded POWER cores too, so that avoids the crappy I2C or SPI interface.

Why are you assuming a large data rate? The connection is extremely slow: JTAG can be thought of as the acoustic coupler of the on-PCB data busses: every station understands what they say and it is robust, but not really fast. We don't need more for this project.

If you are referring to me mentioning Ethernet: there are some participants of this board who would love to design an Ethernet capable backplane that has its own host computer included. You basically plug it into power and network and it starts mining! It was decided to postpone this for the second version, though: once the USB connected version works, adding the CPU on the backplane is straightforward: there is no extra work involved by going over another iteration of boards and that way is safer.

What do you need an Arduino for? The FT2232 does I2C. Edit: Or whatever other USB-chip we decide upon. And I2C has not even been firmly included, yet AFAIK.

The 100mil post system is much worse that the DIMM to hold vertical cards. Only if you have the card parallel to the backplane and use a connector on each end of the elongated board will it be sufficiently stable (my opinion, but plug in a largish card on a few pins and then move it to get a feel).

The text for the pushbutton sounds like hot-plugging. I don't think we want to to there in the first revision: get it wrong and you have fried one or more cards. As for card detection: no mechanics needed: just short two pins in the connector on the FPGA card and the backplane can detect the presence of this connection. And you mentioned the Arduino again. If the backplane becomes "intelligent" in a later revision, I think a bit more powerful CPU would be in order to also handle Ethernet. Or use an even smaller MCU or even a CPLD if all you want is a bit of control logic...