I haven't run SETI in 10.1, so I can't say. However, when the comp is being used, it is definitely better to run in X. I would just run whatever OS you prefer.
For the G4 however, you should definitely run two instances. I'd recommend doing it via cron. You can find out info on how to run it from
here.
Running with cron is good for all your OSX computers, because it starts the command line client up automatically.
If you haven't done so already, you can put your computers on the team proxy. Check out the team seti page, you'll find info on it, and tons of other things.
Team MacNN SETI Page
--Scott