HOw do I know what firmware my Mac has?

Boot OS 9 and run System Profiler. You could also just try installing it (from OS 9). It will tell you if it is not needed.
<http://docs.info.apple.com/article.html?artnum=75130>

If you open the Apple System Profiler under the Apple menu, look at the Production Information. The firmware version is the Boot ROM version. You need to check if your Mac requires a firmware update. Mac OS X: Available firmware updates
http://docs.info.apple.com/article.html?artnum=86117 If an update is required, the update must be installed (using OS 9) BEFORE you install OS X.

You don't need to keep OS 9 on the iMac. However, it doesn't require much hard drive space. The only reason to keep OS 9 is if you intend to run OS 9 applications in the clasic mode. If you want to run in classic mode, OS 9 needs to be updated to OS 9.2.1 or OS 9.2.2 (best).

Does the iMac meet the Tiger requirements - RAM, hard drive space, etc? Mac OS X 10.4 Tiger system requirements http://docs.info.apple.com/article.html?artnum=301341

 Cheers, Tom 😉