I have just entered HDV editing on Mac.

I have a MacPro setup with 2 cinema displays.

Which hardware (PCI card ?) do I need to have the second display sow a HD preview, while the other one shows the timeline and the tools ?
I also have a SD 4:3 videomonitor that I would like to use for showing a live (downscaled) SD preview of my FCP output.

I have read something about AJA Kona LHe and Blackmagic's Decklink HD Pro card and their HD-Link box that can display a preview on a cinema display.

Can someone tell me - what do I need here??

You can use digital cinema desktop preview for live preview on your 2nd cinema display. No additional cards needed.
For normal TV SD preview a Decklink SP will do.

For previewing to a dedicated external monitor you'll need something like an AJA Kona LH or Blackmagic Decklink capture card.

The new Matrox MXO will also do a similar thing

Andrew