Be sure to run no startup items, third party utilities, or other memory intensive programs while running games. They can degrade performance. Don't allow the Classic environment to run in Apple menu -> System Preferences -> Classic. Remove all Apple menu -> System Preferences -> Login Items.
Quit iTunes and iPhoto if running.
Quit web browsers if running, unless grabbing something on the net. Check the disk directory and make your you max your RAM to specifications.
Reduce the screen display colors in Apple menu -> System Preferences -> Displays if none of the above work. If reducing the screen display colors work, your game needs a better graphics card. Only PowerMacs have upgradable graphics cards. iMacs, eMacs, Powerbooks, and iBooks are stuck with the graphics card installed from the factory.
Another way to improve gameplay is to increase the system priority of the game using the public domain software Renicer. It is unknown at this time if Renicer is native to Mac OS X 10.3, and while I've been told by some others that it is no longer necessary under 10.2, I've not heard any bad side effects regarding its use in 10.2. Please submit to my guestbook if you find out it isn't compatible with 10.2 or 10.3 and let me know I should edit this FAQ accordingly.