4 lane Active Silicon card is the best option:
AS D10 PE4H (single SD/HD-SDI input)
AS D20 PE4H (dual SD/HD-SDI input)
1 lane SDI cards are old and being discontinued.

LFG composite card still available:
4 input composite, 1 lane

Blackmagic cards work fine, but can be fussy about the refresh rate.
Some cards inherently frame buffer, although this is less noticeable in 5,1 MacPros.

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