Bensheim,
Here are two resources to get you started:
Macs and Software that will run with Snow Leopard (Mac OS X 10.6.x)Mactracker will show you a lot of information about individual iMac models from Early 2009 through Mid 2011 which can run Mac OS X 10.6.8.
The support status for the iMac (24 inch, Early 2008) is vintage.
The iMac (21.5 inch, Late 2011) shipped with Mac OS X 10.7.
It is my understanding that a Macintosh computer cannot be downgraded from the operating system it shipped with.