Automatic Shutdown

I need to find a way to shut down my Linux PC (Ubuntu 8.04 Server) after it has up & downloaded 100MB.
Anyone has any idea on how to do this?

When I was a kid, I contrived an ingenious contraption with Lego TECHNIC. Using the basic motor, it converted the rotation into repeated up and down thrust that was applied to the `A' button on the Game Boy ADVANCE, which happened to be bound to the procedure swing-whip in the Castlevania game. Making use of my bogus ploy, I could banish multitudes of undead every night and earn experience with little or none effort.

Anyway, OP, I suggest doing something like that.