I tried using a FireFace 800 over Firewire 400 and found a number of problems. I was never satisfied with it. I ended up buying a PCIe Firewire 800 card and then all was well.
I was using a PC, but it was an excellent build, just let down by the Firewire 400.
I recommend checking this out first. RME have some excellent newsgroups accessible via their website. There are some very knowledgeable guys on there that will give you an honest answer... I'd check that out...
Actually, thought I'd find the link for ya instead:
news://news.x-networks.de/rme-audio.forum
Cheers,
Drammy