Given the lack of response, I suggest actually doing it on a single system to see if it will work
. Most of us here have either decided to stay at 1-HD and keep using the old hardware, OR, have made the move to newer systems. The key to experimenting(and I am all for it
) is to always have a good backup plan. For me, that would be to pull the system drive and pop in a fresh drive, load up your dual-boot OSX versions, install some cards and give it a run. If it were me, I would probably update 1 system at a time to either HDX or HD/Native(PCIe) and move things ahead
Given the number of slots in the Mac Pro, would I be correct in guessing you have HD1 or HD2? From my experience with HD3, a native-based system with decent cpu and RAM may turn better performance than HD1 or HD2(indeed, I had projects choke my friend's HD3 rig that ran with ease on my native rig)
BTW, your old IO boxes will likely work dandy with HD/Native cards(my current setup uses a pair of HD IO boxes and an older 192 for extra IO and hardware inserts).