You can capture HD with decklink deck control just fine even if you dont have AGP/PCIe graphics card (in this case you just will not see the live preview on your screen)
It is both – when connected straight (via PCIe) it is capture card – when connected via SDI/Component/Composite but not PCIe then you can use it as converter
But you do need PCIe mac (the newest version of powermacs)
Actually it is a fature for DVI-D, dont know about SDI (because HD sdi would just crop the frame and 2K is not sdi standard)
Why – because I want monitor online editing