Your G4 and your ram is plenty fast, I believe, but what will for sure make all the difference in the world would be if you put in a new video card. Like the previous poster, I have the GeForce 3, and MoH:AA plays very well. Of course I also have the 867 and 1.25 gig ram, so I'm running quite fast.
The old ATI Rage Pro cards are not really gaming cards, so it would be in your best interest to get yourself a new card. Then you will really enjoy playing games on your Mac.
Good luck,
...Bob