I’m actually not using my Drobo for video, but my example holds well especially since you have an older machine.
If you still have the manual for your motherboard (or can find it online), see if it has a block diagram. Many motherboards have a block diagram in the beginning. That’ll show you how things are connected.
The overall throughput you can achieve will depend on where the bottleneck is.
The block diagram will help determine what the “smallest pipe” between the connections are.
For example, if you’re copying from a PATA drive, but the PATA controller is attached to the PCI bus and not built into the bridge (as was sometimes the case with additional storage controllers), then you’ll have PATA traffic on the PCI bus that would compete with the Firewire traffic for a FW800 add-in card.
Another example - I have a workstation-class motherboard with GigE that is not built into the chipset, but an additional NIC that’s wired into the 32-bit PCI bus. Thus, it doesn’t achieve full GigE throughput.
I have a server-class motherboard where the GigE controller is connected differently, so it achieves higher throughput.
All depends on what’s built in and connected where. If your PCI ends up not being dedicated for the FW800 card, then you’re probably going to get about the same speed or less.