The 24 I/O uses a PCI card (Edit: that's included) to interface the rackmount unit to the computer.
The only MOTU interfaces that are FireWire are the 828, 828mkII, 896, 896HD and the Traveler.
Also, the MOTU would be your soundcard. So to speak. You could likely still keep the soundcard that you have to use for games, operating system sounds and whatnot.