Does anyone know what the cost of these 28nm
Artix 7 chips are? I've been looking through and the XC7A350T looks promising with 350k LUs. From the text on the site it looks like they're using this chip to replace the Spartan 6's.
Right now they're not available at any price as far as I can tell; initial samples are meant to be shipping in the first quarter of 2012, with production quantities available who-know-how-long later. This is also wildly off-topic though, given that this thread's about building boards with actual FPGAs that are available now.
Expect to fit at least 4 hashers in the XC7A350T unless Xilinx have done something daft in the design again like they did with Spartan-6, which is entirely possible. I have no idea about clock speeds; estimating that would require a full ISE license.