We use a HP Z600 Dual Quad Core, a BlackMagic HD Extreme 3D Capture Card and Windows Expression Encoder.
http://www.microsoft.com/expression/products/EncoderPro_Overview.aspx
The HPZ600 comes with a higher end Nvidia Graphics Card... not sure what the model is right now but BlackMagic recommends the HP Z Series computers... either a Z800, Z600 or a Z400 for it to work right.
http://www.hp.com/united-states/campaigns/z-workstations/overview.html#zser...
Initailly we had tried to use a regular Core 2 Duo computer but we needed another PCI Express slot on the mother board for a higher end graphics card... this is due to the fact that the Blackmagic card does not show any graphics to a monitor... it depends on the graphic card for that.
The footage is a little bit better then when it was DV and Firewire... and Windows Expression Encoder has a feature called IIS Smooth Streaming...
http://www.iis.net/download/SmoothStreaming
.... and it makes the picture look way better then without it...
Before we were using one camera and now we are using a Kramer Multi Format Switcher for multiple cameras (Sony NX5U NXCAM cameras) and a computer for Power Point as our inputs. From there we output via component HD to a AJA Mini converter to SDI and we then capture the SDI signal to the BlackMagic Card on the HP Z600.
Works flawlessly...I love it... its the sick set up...