You could purchase the Universal Binary disc image for $5.00. I don't know if the old PPC version can use that same disc image to get the game to load, but there may be no harm in having a Universal Binary version of the game. I think the latest update has the UB PPC performance up to par.
There's really not much point trying to play the computer version of Halo with a gamepad anyway, since they took out auto-aim and bumped up the difficulty. Just use mouse+keyboard; it's more precise.