Fresh info from a new integration project we just did in a tv broadcaster.
Display cluster build according to the last specs, including W9100 display card, RAIDs, etc... Win 8.1 Pro x64... latest bios/ drivers/ etc... 24hour live test of the computer prior to the WO installation
We chose Blackmagic Decklink Duo (Dual SDI input) as a capture card since it dual input was the request and it was relatively best card comparing price/ quality.
Although it's written that Decklink Duo works even on a x4 slot in fact it doesn't perform good - there is a black flicker when live feed is provided to the screen (even when only one of the inputs is used). After moving to a faster slot problems are solved (for now and, hopefully, for ever)