Hi,
with Spartan 3 is possible to receive SD-SDI video data (at 270 Mb/s) ; the only think is if you want to correct the jitter you mus put an external VCO to make the best clock recovery.
For the HD-SDI at 1,485 Gb/s you must use a Virtex serie FPGA because you need to use the block Rocket I/O.
The Xilinx have in the web site for each application free reference design with the source code; I tried It and work perfectly; for the HD-SDI I think that the best solution for You is to use the Xilinx VIODC development board in order to develop the application.
Regard
Daniele