However, for a new project you might be better off with some newer board.
(e.g. Digilent ATLYS whit HDMI In and out)
You might have to adapt the codes given in the XAPP for the Spartan6 device, but there may be reference designs available that already cover the usage of the HDMI interfaces.
While it is possible to do some DVI/HDMI I/O with the S3A devices (see limitations on page 1 of xapp460) the faster S6 devices will give you a higher performance which you will need when it comes to using HD video modes.