Good FPGA / Microcontroller development board for real time image processing
In terms of FPGAs, you would be most concerned with high performance DSP on the chip, for that I would recommend a Xilinx Spartan-6 FPGA or a comparable Altera. Digilent makes some low priced development boards that has USB and VGA that might fit the bill.
You will need to program the FPGA with Verilog or VHDL. When programming FPGAs you are describing actual hardware and so it is completely different from coding in C/C++.
Unless someone has ported OpenCV to VHDL or Verilog I doubt it.