The Firewire 800 cards should be backwards compatible (I think; my only experience is with built in, e.g. Macs), but you'll need special cables, which will probably be expensive and hard to find. The 800 uses a 9 pin connector, and the 400 uses 6 or 4 pin connetors (the extra 2 pins carry power).

I am the Doctor, and THIS... is my SPOON!