Hi,
there are a few ways to skin that particular cat. Although I suspect none is very good for your purposes. :)
There are a few standards for which FGPA cards are available. A couple of examples.
ATCA/µTCA:
http://www.mc.com/products/boards/ensemble_mxi205_xilinx.aspx VME:
http://www.acttechnico.com/fpga_computeplatforms.html VME:
http://www.bittware.com/products/hardware/prod_desc.cfm?prodshrtname=gtv6 (Stratix)
There are also a number of PCIe development boards that you could fit into rackmount servers.
SGI was offering RASC modules for their computers.
If you have to go custom, VME is problably the cheapest option if you can live with the low bandwidth.