I am familiar with SimTower (which not only is fun but is an interesting take on a management and economics simulation) and have played it on my iBook G3 300MHz (with OS 9.1) without problems.

A few things to try:

1. Check your monitor's depth. If I recall, SimTower runs best with 256 colors.

2. Make sure there is enough RAM allocated to the program. Try doubling the amount if possible.

3. Start up with your shift key down. SimTower doesn't require any INITs, not even QuickTIme, so it's possible something could be conflicting.

4. Try a clean install.

One final thing to mention--I have the floppy version. I'm not sure if the CD version is different.