Hi,
Can't speak about the Fireface 800, though the consensus on these forums seems pretty positive.
Regards the drives, I've been using 2x
LaCie D2 triple interface drives for a couple of years now, with no problems at all, running up to 100 tracks simultaneous (for testing purposes). The drive in the LaCie box is a Western Digital, and they rackmount.
In short, you run into the transfer-limit of the drive well before the FW800 gives up. I guess you'd only stretch things running multiple drives in some kind of Raid, but AFAIK, that's not currently possible on Windows/FW.
On a desktop, if you're running lots of Sonar audio, and streaming big samplesets to Kontakt simultaneously, I'd probably go with 2x FW400 drives, or 1x FW400, 1x FW800 to keep the datastreams separate from the Fireface i/o
For live use, presumably you'll only need 2x or 4x streams out of the fireface, so you shouldn't max out the FW800 capacity, especially if your PCMCIA card has 2 or more ports, but I'd check the data capacity of the PCMCIA slot anyhow, just to make sure!
Hope this helps
Best Regards
Steve