For the moment I am using a Multibridge Extreme on an Asus AN832SLI Deluxe with an AMDX2 3800+ and an ATI X1900XTX. I had to place my ATI X1900XTX in the first PCIEx16 slot and put the Multibridge beside it in the PCIEx4 slot. Fortunately the hardware although under spec has met my needs for capture (HD via component and Digibeta via SDI). This system spec is not good enough for HD editing.
If I was planning on editing HD I would want dual core Opterons and external RAID storage connected to a PCI-X host bus adaptor.
Maybe I’d choose a Quadro over an ATI card because I would imagine the drivers are more stable for workstation use.
Since you mentioned using another motherboard, I will mention this. Initially I tried using an Asus AN8-SLI Premium motherboard that was lying around and it did not work at all. The Multibridge was not being detected in windows.