Uh, normally, don't you want the game to take up 100% of your CPU? <img border="0" title="" alt="[Wink]" src="wink.gif" />
In OS 9, games running in full screen mode usually take up 100% of the CPU. In OS X, since apps can't completely hog the CPU, the more CPU the game uses, the better. That equates to better performance.
Can you better describe the behavior that is happening? I've run EV Nova on a variety of computers in OS X, including a PowerBook G3 400, and it ran fine on all machines.