Yes, in theory 32-bit PCI bus has up to 133MB/s but that is shared bandwidth and there are various factors contributing to the bottleneck (protocol overhead, other devices like your SCSI card). There are numerous reviews and tests all over the internet besides my real life tests confirming this.
That depends on the MB manufacturer, some choose not to use that feature.
I have an older 865PE MB (Asus P4P800 Deluxe) and it has a 3COM Gigabit controller on the PCI bus.
Well, gigabit and firewire speeds between pc's don't always max out. It all depends on the bus of these network cards. I don't know much about Mac's, but if they're on standard PCI bus you will have a speed limit at around 20MB/s regardless of the theoretical speeds.
You need a higher...