I'm not a mac user, but it sounds like that computer would struggle a bit. You could probably do a handful of tracks but have to limit the plugs, or use a freeze tracks option if your software has it. Give it a try, never know.
It shouldn't be super slow. I think you'll be fine.
I was recording 7 inputs at a time with no problems when my G4 was 350mhz. My recommendation is to have at least 512MB of RAM, but you may be able to get away with less.
