The Upgrade to Aperture 3.3 is a major new release. Aperture 3.3 requires Lion - MacOS X 1.7.4, and will only run on Macs with 64bit Intel Processors. If you upgrade to Aperture 3.3., your Aperture Libraries will be converted to a new format, that is incompatible with earlier Aperture versions. You will no longer be able to open these libraries on your older iMac. Sorry.
and the Aperture 3.3.1 release notes
I have a similar problem, except the fact that my Lion mac already has Aperture 3.3 installed, while my other mac in incapable of running Lion. Hence I seem to be stuck with two incompatible versions of Aperture. Is there no way to downgrade from Aperture 3.3? I have not yet upgraded my Aperture library.
As long as your Aperture library is still version 3.2.4 and you still have an installer for a previous version of Aperture 3, plus the serial numbers, you can just delete the current Aperture version (aftner making a backup) and reinstall.
How did you buy Aperture? If you bought from the App Store and did not keep the installer, you are stuck with the current version (unless you have a bootable clone of your pprevious system), you only can download the most recent version from the App Store.