The peak transfer rate of 32-bit, standard PCI is 133MB/sec. That is greater than the peak transfer rate of Firewire 800, or 1394b, which is just under 100MB/sec. (1394b is just under 800 Mbits/second, or roughly just under 100 MBYTES/sec).
So a standard PCI connection is fast enough for both Firewire 400 and 800.
However, in the architecture of a modern PC, all the PCI slots combine to use a single PCIe connection to the southbridge. If you have other PCI slots filled, your Firewire card will be competing for that bandwidth to the southbridge. If you are like me, and the Firewire card is the only PCI slot filled, then it is fine... but if you have lots of other PCI cards installed, then you might hit a total bandwidth limit. You'll have to make that determination for yourself.
Should you replace your PCI Firewire 400 card? Maybe. You might be able to replace it with a newer PCI card and see some increase in speed, just because there have been advances in the chipsets and drivers. I recommend Unibrain products: Welcome to Unibrain web site - The Firewire (Firewire 800 - IEEE 1394b) Innovators
Their newest PCI and PCIe cards are very good: FireBoard 800-e™ 1394b (Firewire-800) OHCI PCI Express adapter FireBoard-800™ V.2 1394b (Firewire-800) OHCI PCI adapter
Both of these cards can maintain 400Mb and 800Mb speeds, although the PCIe card can do 8 isochronous connections vs 4 for the PCI card. That's only a limiting factor if you have more than 4 devices connected simultaneously.
Also, you don't need a card with both 1394a and 1394b connections. 1394b (800) is down-compatible with 1394a (400). You just need to get a converter cable.