A Firewire 800 (IEEE1394b) card should be backwards compatible with standard firewire devices but wouldn't give you the improved speed.
There are two connectors defined for it, only one of which (bilingual) will also accept the older connectors.
The 800 is Mbps = 60MBps and is independent of FSB speed.
This is all theory taken from Mueller's "Upgrading and Repairing PC's". I've no idea what's happening with it in the real world.