From taking a look at wikipedia, it seems the old PCI 2.0 standard can pass 133MB/sec. That's definitely fast enough to handle as many tracks as you can probably come up with. However that's for the whole PCI bus, not a single slot. What if you're running a Powermac 9600 with 6 PCI slots?

I don't know if the System Bus speed is a factor in this either, because isn't the system bus speed the thing that dictates basically the entire speed of the machine or something? And I remember IIO saying something about RAM speed playing a factor as well.