perhaps you havent updated the game?
http://www.macgamefiles.com/detail.php?item=18390
if you create a user in OSX that has minimal usage eg: just the game as listed for usage then you have created a very optimal user setting for gaming.
Imac G5s are quick but not as fast as there tower counterparts.
after looking at a forum someone suggests that the 5200 geforce might be sluggy when playing this game.
http://forums.macrumors.com/archive/index.php/t-92791
If no-one gives your an answer to a question you have asked, consider:- Is your question clear.- Did you ask nicely.- Are you showing any effort to solve the problem yourself