The Mojo is a FireWire 400 device as it does not need more than that to do what it does. WHat I don't know is if supported on an 800 bus. I will need to check.
I just ordered a new computer and I specified a Firewire 800 card. I'm planning on getting also an Iogear GHD835C250 for maximum speed, storage and flexibility.
My coming computer is a P4 3.4Ghz Extreme. Radeon 9800 Pro (256Mb). 2GB RAM. Audigy 2 ZS. And more. I hope this will run Avid Xpress Pro and Mojo!
Avid says Mojo must be on a 400 board, it does not work on an 800. Mojo must also be alone on it's own board. so use the 800 for your HD, and put in another 400 to interface w/ Mojo. also keep in mind that different cards should be on different PCI busses. the Avid website has all this info.
i'm running Pro w/ Mojo on a Dell 8300 speced w/ a 3.0 GHz P4, 1GB RAM, 9800 XT (256), Audigy 2, internal SATA media drive. it's not like my work system on an HP wx8000 dual Xeon, but it functions.