Wondering if a firewire 400 card could benefit by a PCI-E connection or is that just for people that don't have a PCI slot? PCI-E cards seem to be a lot more expensive.
More aboutadding firewire card
The PCIe bus is faster than the standard PCI, and the cost of an adapter card ranges from 10$(PCI) to 20$+(PCIe). If the extra 5-10$ is going to ill your budget get the PCI, otherwise PCIe is the way to go...
The standard PCI slot has ~100MB/s usable bandwidth shared between all devices on the bus, which is about twice as much as FW400 so there should be no problems unless you have other high-bandwidth devices on the PCI bus.