Barry,
Since you didn't specify which MAC systems you are using - it is hard to answer your question, specifically.
I would suggest that you look at adding an internal Serial ATA RAID 0 into your MAC. DVCPROHD is 100 Mbps or about 12.5 Mbytes per second and is capable of being captured or edited on a single Firewire or internal disk, but if you are going to be using 2 or more streams, you will need at least a 2 stripe RAID.
A setup like this is actually capable of at least 2 streams of uncompressed SD.
Better to have more bandwidth that you need.